Lane Garrison -- 'Visible Marks' Found on Alleged Domestic Violence Victim

Lane Garrison is under investigation for allegedly hurting his girlfriend ... sources tell TMZ.

Cops won't confirm who the alleged victim is ... but we're told Lane and girlfriend Ashley Mattingly got into another big verbal blowout Saturday night. Cops got a 911 domestic violence call and responded to the scene. When they arrived, they saw "visible marks" on Mattingly.

Sources say the verbal argument escalated to a physical brawl.

By the time cops arrived, the former "Prison Break" star had split. Now he's under investigation for felony domestic violence.

On the video (above) ... you see Mattingly speaking to police outside Lane's apt.

Earlier in the day, Mattingly tweeted, "sooo happy to be single again from @LaneGarrison9 bye bye :) hahahaha." When someone called her out for being mean, she tweeted back, "no drunk a** is trying to brake into my apt!"