Kroy Biermann Jr. Arrest

The police investigating 'Real Housewives of Atlanta' alum Kim Zolciak's son Kroy Biermann Jr. for an alleged sexual battery say they went out to the scene of the alleged crime to obtain evidence ... and they saw him and the alleged victim on video.

According to court docs, obtained by TMZ, the Alpharetta Police drove to the gym where Kroy Jr. allegedly forced a girl from his school to put her mouth on his penis. She also claims he digitally penetrated her despite telling him no numerous times.

The officer says he reviewed security footage from the gym ... showing the alleged victim arriving and later hanging out with Kroy Jr.

'Real Housewives of Atlanta' alum Kim Zolciak is firing back at allegations that her son Kroy Jr. sexually assaulted a classmate inside a gym ... claiming he even took a lie-detector test to prove it never happened.

The reality star tells TMZ ... "The allegations being made against my son are nothing short of reckless and deeply troubling. He categorically denies ever being involved with this woman or engaging in the conduct she has alleged."

Kim claims her son "voluntarily took a polygraph examination regarding these allegations and passed."