Boulder County fire investigators have ruled out natural causes for the Gregory Fire, which scorched between 2 and 3 acres on Boulder Open Space and Mountain Parks land near the First Flatiron on Wednesday.

“They believe it is human-caused,” Boulder County Sheriff’s Office Cmdr. Mike Wagner said on Thursday evening. “The investigation is still ongoing.”

He said sheriff’s investigators are interviewing witnesses and looking for anyone else who might have information regarding the cause.

Wagner added that investigators determined the fire originated just below Crown Rock on the eastern slope near the 2000 block of Flagstaff Road and was contained to city open space.

Boulder firefighters were able to get the fire fully contained on Thursday morning, more than six hours sooner than previously anticipated. Fire crews remained on scene overnight into Thursday to monitor the area.

The fire did not threaten any homes or structures and no injuries associated with the fire were reported.

Firefighters worked in steep, rocky terrain throughout Wednesday afternoon and evening, and police temporarily shut down Flagstaff Road. Fire officials called for air support but canceled it after determining fire crews had a handle on the blaze.

Trails in the area, which had been shut down, reopened on Thursday.

Staff writer Mitchell Byars contributed to this report.

John Bear: 303-473-1355, bearj@dailycamera.com or twitter.com/johnbearwithme