Denver police today reminded people to watch their backs downtown, because groups with “predatory behavior” are targeting strangers alone on the streets.

Fifteen attacks and robberies have been reported between Aug. 1 and Nov. 9. The incidents do not seem to be racially motivated, police said this afternoon.

One person might have been involved in several of the cases involving groups of between two and eight attackers. No arrests have been made despite a $2,000 reward.

Six of the attacks have happened on the 16th Street Mall downtown, five on Market Street in Lower Downtown and others on 14th, 15th and 18th streets.

In many cases, attackers initiate contact with a stranger on the street, which quickly escalates it into a confrontation, before others join in the assault and rob the victim.

Police said some witnesses might have taken pictures during the attacks.

“We are asking those people to come forward and provide us with the footage,” Denver police stated in a media release. “This new information could assist investigators and prevent others from possibly being victimized.”

Anyone with information can call Crime Stoppers at 720-913-7867, send a text to CRIMES (274637) with the title DMCS, or send an e-mail to metro-denvercrimestoppers.com.

Police advised people who walk alone downtown:

Be aware of your surroundings.

Walk only on busy, well-lit streets, even if it makes the trip a little longer.

Avoid shortcuts through parking lots, alleys or parks.

If you feel you are being followed, cross the street or begin walking in a different direction. Walk toward lighted, more populated areas.

If a vehicle is following you, turn and run in the opposite direction. The driver will have to turn around before continuing to follow you, buying you time to make it to safety.