An agent of the Drug Enforcement Administration was killed when a passenger, who also died, opened fire when police were carrying out a regular check for illegal firearms and other contraband onboard an Amtrak train in Tucson, Arizona.

According to Tucson Police Chief Chris Magnus, one DEA special agent is in critical condition and a Tucson police officer operating on the DEA task force is in stable condition at a hospital.

Several agents entered the double-decker train and made contact with two passengers on the second level, ABC News reported.

Magnus said as police were holding one individual, a second person pulled out a pistol and started firing on the officers.

The incident, which sent passengers scampering for safety, took place just after 8 a.m. at the station in the city's downtown, Monday.

One of the cars had been boarded by a regional task team of DEA agents and Tucson police officers for a routine search for illegal goods and narcotics. Magnus said, it happens all the time at transit terminals.

The alleged shooter locked himself in a lower-level restroom after exchanging bullets with the police, officials said. The alleged shooter was subsequently found dead, however the manner of his death remains unknown, authorities added.

According to Amtrak, none of the 137 passengers or 11 staff members were injured. Amtrak representative Jason Abrams also said there were no reported injuries to the passengers or crew.

The train was headed to New Orleans from Los Angeles and arrived in Tucson around 7:50 a.m. local time.

One of the passengers was quoted by CNN as saying that said he didn't see or hear anything, but he was in a lounge car when people ran through "yelling, 'Shots fired!"

"It's extremely horrible, and we're all just coming to terms with just how terrible a loss this is," Magnus said. "However, I'd want to focus on the officers' very courageous efforts on the scene."

The other suspect was apprehended. It's unclear what accusations he's facing or how he's connected to the second man.

The Federal Bureau of Investigation is taking the lead in the investigation, the bureau and the Tucson Police Department, said.