Steve-O -- Under Investigation for 'Deadly Conduct'

Steve-O's latest stunt -- backflipping from a bridge into the shallow, San Antonio River -- could land him in jail for the crime of "Deadly Conduct."

Steve made the jump last Friday ... a very risky proposition given the water below is only 4' deep.

Cops threatened to cite him but didn't ... however, law enforcement sources now tell us the "Jackass" star is under criminal investigation.

So, you ask, what is the crime of "Deadly Conduct." Under Texas law, someone who engages in reckless conduct that could place someone else in danger of great bodily injury, then it's considered deadly conduct ... a misdemeanor punishable by up to a year in jail.