AUSTIN, Texas -- A store of ammonium nitrate is what exploded April 17 at a Central Texas plant, killing 14 people, injuring hundreds and devastating an adjoining town.

The finding was expected, and officials had said they were focusing their investigation on the explosive chemical used in many fertilizers, said Rachel Moreno, spokeswoman for the Texas State Fire Marshal's Office. A spot where the ammonium nitrate was stored is now a 90-foot-wide crater, Moreno said Monday.

However, the ignition source for the explosive chemical remained undetermined Monday. Findings on the cause of the blast on the outskirts of the small town of West initially had been expected Friday. However, the investigation will take one to two extra weeks to complete, with dozens of investigators combing through plant wreckage and the adjoining wrecked neighborhood, Moreno said.

Also, federal emergency officials have begun offering shelter for West residents whose homes were destroyed or severely damaged. About 70 homes were damaged or destroyed.

A statement from the Federal Emergency Management Administration said the transitional sheltering assistance was requested by Texas state officials. It would allow those whose homes were left uninhabitable by the blast to stay for a limited time in a hotel or motel at government expense. Meals, telephone calls and other incidental charges are not covered, and applicants are responsible for any lodging costs above the authorized lodging costs, according to the statement. Eligible applicants are being notified.

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  • Fertilizer Plant Explosion

    In this Instagram photo provided by Andy Bartee, a plume of smoke rises from a fertilizer plant fire in West, Texas on Wednesday, April 17, 2013. An explosion at a fertilizer plant near Waco Wednesday night injured dozens of people and sent flames shooting high into the night sky, leaving the factory a smoldering ruin and causing major damage to surrounding buildings. (AP Photo/Andy Bartee) MANDATORY CREDIT: ANDY BARTEE

  • Fertilizer Plant Explosion

    The remains of a home burn early Thursday morning, April 18, 2013, after a fertilizer plant exploded Wednesday night in West, Texas. The massive explosion killed as many as 15 people and injured more than 160, shaking the ground with the strength of a small earthquake and leveling homes and businesses for blocks in every direction. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    Firefighters use flashlights early Thursday morning, April 18, 2013 to search a destroyed apartment complex near a fertilizer plant that exploded Wednesday night in West, Texas. The massive explosion killed as many as 15 people and injured more than 160, shaking the ground with the strength of a small earthquake and leveling homes and businesses for blocks in every direction. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    Emergency workers evacuate elderly from a damaged nursing home following an explosion at a fertilizer plant Wednesday, April 17, 2013, in West, Texas. An explosion at a fertilizer plant near Waco caused numerous injuries and sent flames shooting high into the night sky on Wednesday. (AP Photo/ Waco Tribune Herald, Rod Aydelotte)

  • Fertilizer Plant Explosion

    A person looks on as emergency workers fight a house fire after a nearby fertilizer plant exploded Wednesday, April 17, 2013, in West, Texas. A massive explosion at the fertilizer plant near Waco on Wednesday night injured dozens of people and sent flames shooting into the night sky, leaving the factory a smoldering ruin following a blast that damaged buildings for blocks in every direction. (AP Photo/Waco Tribune Herald, Rod Aydelotte)

  • Fertilizer Plant Explosion

    Shown are the remains of a fertilizer plant destroyed by an explosion in West, Texas, Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    Emergency workers assist an elderly person at a staging area at a local school stadium Wednesday, April 17, 2013, in West, Texas. An explosion Wednesday night at a fertilizer plant near Waco sent flames shooting high into the night sky, leaving the factory a smoldering ruin, causing major damage at nearby buildings and injuring numerous people. (AP Photo/Waco Tribune Herald, Rod Aydelotte)

  • Fertilizer Plant Explosion

    Firefighters conduct a search and rescue of an apartment destroyed by an explosion at a fertilizer plant in West, Texas, Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    A destroyed car sits as firefighters conduct a search and rescue of an apartment complex destroyed by an explosion at a fertilizer plant in West, Texas, Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    Firefighters conduct a search and rescue of an apartment destroyed by an explosion at a fertilizer plant in West, Texas, Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    Firefighters conduct a search and rescue of an apartment building destroyed Wednesday by an explosion at a fertilizer plant in West, Texas, Thursday, April 18, 2013. The massive explosion at the plant killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    An injured elderly person is assisted by two young males as a nursing home is evacuated after an explosion at a nearby fertilizer plant Wednesday, April 17, 2013, in West, Texas. (AP Photo/Waco Tribune Herald, Rod Aydelotte)

  • Fertilizer Plant Explosion

    Firefighter conduct search and rescue of an apartment destroyed by an explosion at a fertilizer plant in West, Texas, Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    The remains of a fertilizer plant sits destroyed after an explosion in West, Texas, Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    The remains of a fertilizer plant sits destroyed after an explosion in West, Texas, Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    Firefighter conduct search and rescue of an apartment destroyed by an explosion at a fertilizer plant in West, Texas, Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    The remains of the the West Fertilizer Co. plant smolder in the rain on Thursday, April 18, 2013, in West, Texas. A massive explosion at the plant killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/Houston Chronicle, Smiley N. Pool)

  • Fertilizer Plant Explosion

    Elderly persons from a nearby nursing home are triaged in a parking lot before being moved to a school stadium following a fertilizer plant explosion Wednesday, April 17, 2013, in West, Texas. The explosion near Waco Wednesday night injured dozens of people and sent flames shooting high into the night sky, leaving the factory a smoldering ruin and causing major damage to surrounding buildings. (AP Photo/Waco Tribune Herald, Rod Aydelotte)

  • Fertilizer Plant Explosion

    Firefighters walk next to a destroyed apartment complex near a fertilizer plant that exploded earlier in West, Texas, in this photo made early Thursday morning, April 18, 2013. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    Persons are seen pushing wheel chairs in front of a damaged nursing home following an explosion at a nearby fertilizer plant Wednesday, April 17, 2013, in West, Texas. An explosion at a fertilizer plant near Waco caused numerous injuries and sent flames shooting high into the night sky on Wednesday.(AP Photo/ Waco Tribune Herald, Rod Aydelotte)

  • Fertilizer Plant Explosion

    Firefighters use flashlights to search a destroyed apartment complex near a fertilizer plant that exploded earlier in West, Texas, in this photo made early Thursday morning, April 18, 2013. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    A fire smokes near a fertilizer plant that exploded earlier in West, Texas, in this photo made early Thursday morning, April 18, 2013. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    Firefighters us flashlights to search a destroyed apartment complex near a fertilizer plant that exploded earlier in West, Texas, in this photo made early Thursday morning, April 18, 2013. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    A fire still burns in a apartment complex destroyed near a fertilizer plant that exploded earlier in West, Texas, in this photo made early Thursday morning, April 18, 2013. (AP Photo/LM Otero)

  • Fertilizer Plant Explosion

    Smoke fills the air after a fertilizer plant explosion Wednesday, April 17, 2013, in West, Texas. A massive explosion at the fertilizer plant near Waco on Wednesday night injured dozens of people and sent flames shooting into the night sky, leaving the factory a smoldering ruin following a blast that damaged buildings for blocks in every direction. (AP Photo/Waco Tribune Herald, Rod Aydelotte)

  • Fertilizer Plant Explosion

    A fire fighter searches a nursing home that was damaged after an explosion at a neary by fertilizer plant Wednesday, April 17, 2013, in West, Texas. (AP Photo/ Waco Tribune Herald, Rod Aydelotte)

  • Fertilizer Plant Explosion

    Map locates fertilizer plant explosion near West, Texas

  • Fertilizer Plant Explosion

    Bobby Rios, right, helps clean up the shattered display windows at Treasure on the Tracks in the central business district of West, Texas on Thursday April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. The explosion that struck around 8 p.m. Wednesday, sent flames shooting into the night sky and rained burning embers and debris down on shocked and frightened residents. (AP Photo/The Fort Worth Star-Telegram, Ron T. Ennis) MAGS OUT; (FORT WORTH WEEKLY, 360 WEST)

  • Fertilizer Plant Explosion

    Maria Galvan looks for damage to merchandise inside the West Thrift Shop in West, Texas on Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. The explosion that struck around 8 p.m. Wednesday, sent flames shooting into the night sky and rained burning embers and debris down on shocked and frightened residents. (AP Photo/The Fort Worth Star-Telegram, Paul Moseley) MAGS OUT; (FORT WORTH WEEKLY, 360 WEST)