UPDATE: ARTICLE:
A Drug Enforcement Administration special agent was killed Monday when a passenger, who also died, opened fire as officers were inspecting a train.
amp.azcentral.com
Some eyebrow-raising questions in that publication though...
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A regional task force of DEA agents and Tucson police officers had boarded one of the cars to do a typical check for illegal money, weapons and drugs. It's a common occurrence at all transit hubs, Magnus said
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I am not sure how these protocols go, perhaps somebody with more knowledge can clarify, but I am almost certain that dedicated teams of DEA agents and local police do not just conduct random sweeping searches of intra-USA transportation unless there is a tip that illegal activity is confirmed to be taking place. I have rode Amtrak numerous times in the past and have only seen Port Authority Police at NYC's Penn Station. And they were just guarding the station, platforms, and tunnels... Going by the details of the boarding, arrest of one of the perps, and the second perp opening fire, it appears that LE had already gotten a solid lead that this particular train has contraband and traffickers on board... Probably cartel affiliated.
Fucked up situation through and through as train was filled with passengers too.