A fourth suspect in the beating on Monday of a mentally disabled man on the Downtown Crossing Station Orange Line platform turned himself in to Transit police detectives today, and authorities are searching for a fifth person, officials said.

Germial Johnson, a 28-year-old homeless man, surrendered at about 3 p.m. in Dorchester, according to MBTA spokesman Joe Pesaturo. Johnson is scheduled to be arraigned on assault charges tomorrow in Roxbury District Court, Pesaturo said.

Also this afternoon, Transit police announced that an arrest warrant has been issued for a fifth suspect, Ivan Bacai, 23, of Malden. Anyone with information on his whereabouts can contact police at 617-222-1212, or by texting 873873.

Members of the public can also contact police via the MBTA See Say app for their phones.

Earlier today, a 29-year-old Jamaica Plain woman was arraigned on charges that she participated in the attack, which occured shortly before 12:37 a.m. on Monday, prosecutors said.

Maryanne Hamilton had her bail on an unrelated case in Brookline revoked. A judge set bail on the subway assault charges at $4,500 cash and ordered her to stay away from the victim and MBTA property. Her next court date is Sept. 25, Suffolk District Attorney Daniel F. Conley’s office said in a statement.

Hamilton was allegedly one of four people who attacked Carlito Rezende, a 40-year-old man with a mental disorder who has a restricted ability to communicate. Two others have already been arrested and were arraigned on Tuesday, the Globe reports today.

Hamilton was arrested Tuesday when she was spotted in the courthouse hallway after the arraignment of the two others, Karen Akiba, 22, of Cambridge, and Bryon Lashus, 32, of Watertown.

All of the suspects face charges of assault and battery on a disabled person and assault and battery with a dangerous weapon.