Propane tanks and a pipe bomb found in a Denver mall's food court after a fire may be linked to the 12th anniversary of the Columbine High School massacre.
The Southwest Plaza Mall was evacuated after the fire. No one was injured and the bombs didn't explode.
One official called the location and timing of the incident "disturbing."
The mall is about three kilometres from Columbine High School, where two student gunmen killed 13 people and themselves on April 20, 1999. Unexploded pipe bombs and a propane tank with explosives attached were found in the school after the shootings.
"The fact that has happened on April 20, 12 years later, near the school and with similar devices is very disturbing," Jefferson County sheriff's spokeswoman Jacki Kelley said. "It's something that can't be ignored."
Through the years, students across the country have been accused of threats and incidents modeled after Columbine.
FBI spokesman Dave Joly said authorities have identified a possible person of interest, but no details were immediately available. It's too early to speculate whether the incident was an act of terrorism, he said.
Sheriff's officials said there are usually about 6000 to 10,000 people in the mall around noon each day. Shoppers and about 300 mall employees left after the fire.
About 25 schools were on lockout as a precaution, meaning access is restricted to one point, according to The Denver Post. Classes had been cancelled at Columbine High School in remembrance of those killed in the shootings.
Eric Harris and Dylan Klebold wounded several students and killed 12 and one teacher before shooting themselves.