Federal investigators have determined that the West Oakland three-alarm fire that demolished a construction site, melted parked cars and shut down transbay BART service for over 12 hours last week was caused by arson.

Officials suspected that arson may have been to blame for the damaging blaze. According to SF Weekly, a security guard told officials that he had been harassed by three people just before the fire. Later, he saw them behind the building and thought one might have had something in his hand.

On Friday, Special Agent Christian Hoffman of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives told the San Francisco Chronicle that investigators have evidence proving the fire was indeed "man-made."

However, Hoffman would not reveal what the evidence was, as the incident is still under investigation.

"Only two people know what started the fire: the ATF and our partners, and the suspect that committed the crime," said Hoffman to the Chronicle. "We don't want to reveal anything at this point. We're going to continue following up on leads that we're receiving."

Although no one was hurt, the fire caused $25 million in damage, according to the San Jose Mercury News. The blaze also badly damaged BART insulators and communication and electrical equipment, prompting the shutdown.

Take a look at footage from the disaster in the video below: