BOULDER, Colo. -- A Boulder-based rape crisis center said calls to its hotline are up 11 percent this year compared to last year. The Boulder Daily Camera reports that increase is unusually steep for the service that typically sees minimal changes from year to year.

That news comes after a series of six random assaults in Boulder. Six women have been attacked since Halloween.

Police have not made any arrests. Officers told 7NEWS the descriptions of the attackers differ, so police have not determined if the assaults are connected.

Officers have increased patrols on University Hill.

Officials with Moving to End Sexual Assault said their rape hotline is hearing from women who have been assaulted and friends of victims who want advice.

So far this year, MESA has received 896 calls to its hotline, up from the 804 it received all of last year, according to the Boulder newspaper.

“We have to remind people that 80 to 85 percent of the time, it’s not a stranger,” said Janine D’Anniballe, executive director of MESA