DENVER (Reuters) - A busy shopping mall near Columbine High School was evacuated on Wednesday after authorities responding to a small fire at the retail complex found two propane tanks and a pipe bomb, officials said.

Twelve years to the day after two Columbine High School students shot dead a teacher, 12 students and themselves on April 20, 1999, the devices were discovered at Southwest Plaza Mall, about a mile from Columbine.

Jacki Kelley, spokeswoman for the Jefferson County Sheriff's Office, said the security scare began when a small blaze broke out in the mall's food court around noon on Wednesday.

Firefighters arriving on the scene discovered the propane tanks "at the origin of the fire," and police ordered an estimated 10,000 shoppers and mall employees out of the complex, Kelley said.

Bomb squads later uncovered the pipe bomb nearby as they combed through the sprawling plaza with explosives-detecting dogs, she said.

Among the arsenal that Columbine assailants Dylan Klebold and Eric Harris brought to school during their assault in 1999 were pipe bombs and propane tanks fashioned into bombs.

The similarity of devices found at the mall to the explosives in the school attack was not lost on investigators, Kelley said.

"It's very disturbing that this happened today of all days," Kelley said.

FBI agents called to the scene were treating the bomb placements as "a case of domestic terrorism," Kelley said.

FBI spokesman Dave Joly later told reporters that investigators believe the pipe bomb was intended to trigger a larger explosion of the propane tanks.

Kelley said the bomb fell apart while explosives technicians were handling the device as they prepared to detonate it, and it was "rendered safe."

Investigators reviewed videotapes from surveillance cameras for clues, and later released two still images from the tapes showing a gray-haired man with a mustache and baseball cap they described as a "person of interest."

The FBI asked for the public's help in locating the unidentified man, who was captured in one photo near a door by a stairwell, carrying a plastic grocery bag in one hand.

Columbine cancels classes each year on the anniversary of the massacre there. But other schools in the area were placed on lock-down during Wednesday's bomb scare at the mall as a precaution until the all-clear was given, Kelley said.

The mall will remain closed until the investigation is complete.

Discovery of the pipe bomb came a day after police in Colorado Springs, about 50 miles to the southeast, confronted a teenage boy who admitted posting "Columbine-style threats" against his high school on his Facebook account.

A police spokesman said the Palmer High School ninth grader told officers who visited his home Tuesday that the threats were meant as a joke, and he apologized, along with his family.

The student, whose name was not released, also agreed to stay home from school on Wednesday. Police patrols and security at the school were stepped up for the day, police said.

(Reporting by Keith Coffman and Steve Gorman; Editing by Dan Whitcomb, Greg McCune)