This in addition to the credible threat of the threat against police, and possible teaming up of Bloods and Crips to attack the police.

Police warned the mall, and shut down buses and subway, but apparently failed to marshall sufficient police forces.

Via Baltimore Sun:

The incident stemmed from a flier that circulated widely among city school students via social media about a “purge” to take place at 3 p.m., starting at Mondawmin Mall and ending downtown. Such memes have been known to circulate regularly among city school students, based on the film “The Purge,” about what would happen if all laws were suspended.

The flier included an image of protesters smashing the windshield of a police car Saturday during a march spurred by the death of Freddie Gray, a 25-year-old man who suffered a spinal cord injury earlier this month after being arrested by city police.

Kowalczyk would not speculate on whether the incident was related to Gray’s death.

Mayor Stephanie Rawlings-Blake activated the city’s emergency operation center just before 6 p.m. to coordinate response to the riots.

Police were preparing for rioting to make its way downtown, with officers in helmets and carrying shields stationed at Lexington Market and the Inner Harbor. Maryland State Police sent 40 troopers to the city, said Keiffer Mitchell, a top aide to Gov. Larry Hogan. Several other jurisdictions, including Baltimore and Howard counties, sent officers to assist.

All but one gate to Oriole Park at Camden Yards was closed, with a game set to begin at 7:05 p.m. But team officials postponed the game less than an hour before first pitch was scheduled.