A DEA agent was killed and others were injured in a shooting on an Amtrak train in Arizona

• A shooting broke out on an Amtrak train in Tucson, Arizona, on Monday.
• A DEA agent was killed in the shooting, and two other law enforcement officers were injured.
• The passenger who opened fire also died, police said.

A Drug Enforcement Administration officer was killed in a shooting on an Amtrak train in Arizona on Monday, Tucson Police said.

Tucson Police Chief Chris Magnus said in press conference that the passenger who opened fire is also dead, and a second DEA agent and a Tucson police officer were injured in the shooting.

Magnus said the shooting happened as DEA officers were performing a routine check of the train for "illegal guns, money, drugs" at Tucson's downtown train depot.

He said that the passenger who died had opened fire as another man was being detained by DEA agents.

It remains unclear if the passenger was killed by law enforcement or if he died by suicide, Magnus said.

"It's very horrific and we're all just coming to terms with just how terrible a loss this is," Tucson Police Chief Chris Magnus said. "But I also want to reflect on the really heroic actions of the officers at the scene. They literally ran towards the danger, into the car, where there was an active shooting situation going on."

An Amtrak spokesperson told CBS News the train was carrying 137 passengers and 11 crew members and had been traveling between Los Angeles and New Orleans when the shooting broke out.

An investigation into the shooting is still ongoing.