Chris Brown's Friend, Lowell Grissom Jr., Under Review for Alleged Rapes

Chris Brown's friend, Lowell Grissom Jr., who's been accused of raping a woman during a party at the singer's house ... is now officially on the L.A. County District Attorney's radar.

Law enforcement sources tell TMZ ... LAPD has sent the case to the D.A.'s Office. You'll recall, an anonymous woman claims Grissom, Brown and another woman held her captive in a room at Chris' house ... and that Grissom raped her twice.

The woman, who's suing as a Jane Doe, said she reported the rapes to police. LAPD's completed its investigation, but our sources tell us the evidence is thin, and it will be hard to prove a lack of consensual sex.

Still, it's been presented to the D.A. to make a final decision about charges.