One Drug Enforcement Administration agent was killed and two other officers were injured in a shooting on an Amtrak train that was stopped in Tucson, Arizona, authorities said.

Officers boarded the train Monday morning to perform a routine check for illegal guns, money and drugs and encountered two people on the second level of the double-decker Amtrak car, Tucson police said.

One individual was detained, but a second person revealed a handgun and opened fire, police said.

One DEA agent was killed and another DEA agent is in critical condition, police said.

A Tucson police officer who was on the platform heard the gunfire and ran onto the train, at which point he was shot, police said. That officer is in stable condition, police said.

After exchanging rounds with police, the suspected gunman barricaded himself in a lower-level bathroom, authorities said. It was later determined that the suspected gunman died, though it is not clear how he died, police said.

There were no reports of injuries to the 137 passengers or 11 crew members, Amtrak said.

The train was en route from Los Angeles to New Orleans and arrived in Tucson at 7:40 a.m. local time, Amtrak said.

ABC News’ Nicholas Kerr contributed to this report.