Shots were reported outside Camp Shelby, a military post in Mississippi, for the second straight day, federal and state military officials said on Wednesday.
No service members were reported injured in the incidents.
State police said they were questioning a white male suspect and were using dogs to check his maroon truck for explosives. The man has said that he has no weapons and that his truck backfired, according to the Mississippi Bureau of Investigation.
Soldiers training at Camp Shelby reported that shots were fired into the air around 8 a.m. on Wednesday in the same area where gunfire was reported on Tuesday, Lieutenant Colonel Christian Patterson, a spokesman at the Mississippi Military Department, said in a statement.
The joint forces training center is secure but operating at a heightened state of alert, he said.
On both days, the shooter was described to police as a white man in a red pickup truck who appeared to have fired from a road outside the base, a training center covering more than 134,000 acres near Hattiesburg, Mississippi, 110 miles north of New Orleans.
The incidents follow last month's killing of five U.S. servicemen at a military training center in Chattanooga, Tennessee, in an attack by a gunman who was later shot dead.
That rampage is being investigated by the FBI as an act of domestic terrorism, and it has heightened concerns about attacks on U.S. military targets by homegrown "lone wolves" who may be inspired by Islamic State or other militant groups.
Mississippi Governor Phil Bryant on Wednesday pledged to deploy all resources investigate the reports and make an arrest if a shooter is found.
"The soldiers at Camp Shelby and across the state can and should take appropriate steps to defend themselves as necessary," he said in a statement.
Camp Shelby is hosting a combat training exercise involving more than 4,600 soldiers from the U.S. National Guard and Army Reserve units, Patterson said.
(Writing by Letitia Stein and Karen Brooks; Editing by Paul Thomasch, Mohammad Zargham and Susan Heavey)