Washington Redskins safety Montae Nicholson is cooperating with authorities who are investigating the death of a 21-year-old woman, TMZ Sports has learned.

Here's what we know ...  law enforcement has confirmed a "death investigation" is underway after an unconscious and unresponsive woman was brought to an E.R. in Ashburn, Virginia on Thursday around 1:30 AM.

The victim was pronounced dead in the emergency room. We're told officials believe the woman died from a drug overdose.

Cops tell us ... "The two males who brought her to the emergency room immediately left the area but have since been identified and located."

Multiple sources connected to the situation tell us one of the men is Montae Nicholson.

Nicholson has NOT been arrested or charged with a crime. However, cops say the case is being actively investigated by the Loudoun County Sheriff's Office and it's likely they'll speak with Nicholson as part of the probe.

Nicholson was NOT at Redskins practice on Thursday and we're being told his absence is due to this situation and not his ankle injury.

We reached out to the Redskins and a rep told us, "We’ve been made aware of the tragic event and extend our deepest sympathies to the family who lost a loved one."

"Our team has alerted the league office and is cooperating with local authorities. We fully support law enforcement and respect their process as they investigate the matter."

Nicholson was arrested back in December 2018 after cameras caught the NFL player in a late-night street brawl with a civilian.

The video, obtained by TMZ Sports, shows Nicholson (in a red hoodie) wrestling with a man in a jean jacket before he eventually lands a violent knockout blow.

Montae was arrested and charged with assault and battery and public drunkenness ... but prosecutors eventually dropped the case citing insufficient evidence.