Jameson Williams Traffic Stop Under Investigation After NFLer Dodges Gun Arrest

4:08 PM PT -- Detroit PD tells TMZ Sports ... "This investigation continues to evolve, and new information has come to light that we are actively addressing."

"We take every investigation seriously, especially when new allegations and facts emerge, and we are committed to acting in a swift and transparent manner."

The statement continued ... "Once new facts are confirmed, we make the necessary changes in our decision-making process."

"Let it be clear: no one in the City of Detroit is above the law. Anyone who commits a crime will be held accountable, regardless of their position or status."

Detroit Lions receiver Jameson Williams narrowly escaped being arrested for carrying a concealed weapon earlier this month ... and police are now investigating why he was let off the hook.

23-year-old Williams was riding around Detroit with his brother after midnight on Oct. 8 ... when the vehicle was pulled over for a traffic violation.

During the stop, Williams' brother told cops there were two guns in the vehicle ... and one was located under the NFLer's passenger seat.

According to WXYZ in Detroit, one weapon was registered to Williams' brother, who had a concealed pistol license ... the other was registered to Williams, who did not have a CPL.

Williams was alerted he would be taken into custody for carrying a concealed weapon ... but according to the outlet, he tried to use the "I'm a Lions player" card several times -- which didn't really impress the officer.

Williams' brother also tried to take responsibility for both guns ... but the former Alabama star later admitted one was his, claiming it was "for protection" because he lives in Detroit.

The receiver was ultimately placed in cuffs and put in the back of a squad car ... and while it seemed like he was well on his way to jail, it all changed when a supervising sergeant showed up.

The supervisor -- who, according to the outlet, had a Lions-themed background on his phone -- seemed upset with the officers on the scene ... saying, "I'm so mad at you two."

After he made a few calls, the supervisor was heard on the phone suggesting special accommodations for Williams' jail arrival ... that is, before he got a call from a lieutenant, who seemingly gave Sarg the answer he was searching for.

"Okay, beautiful. I'm good to let him go?" the sergeant asked, according to the outlet. "You're a (expletive) hero. Thank you so much."

Once the supervisor hung up, Williams was free to go.

The whole traffic stop is now at the center of an internal investigation ... and Detroit police have since submitted a warrant request to the Wayne County Prosecutor's Office for Williams' arrest for carrying a concealed weapon in a motor vehicle.

The warrant request is currently under review.

The Lions addressed the matter in a statement ... saying Williams alerted the organization immediately after it happened, and the league is aware of it as well.

Williams -- currently suspended for violating the league's performance-enhancing drug policy -- has 17 catches for 361 yards and three touchdowns this season.