A nationwide manhunt is under way Monday for a man wanted in connection with last week's attempted bombing at a Colorado mall.   

Authorities say they're searching for 65-year-old Earl Albert Moore, who has an “extensive criminal background," as the main suspect involved in an attempted bombing Wednesday at the Southwest Plaza Mall in Littleton, Co.

The explosives did not detonate, but the incident alarmed authorities and residents because the plan took place on the 12th anniversary of the shootings at Columbine High School in Littleton. 

But investigators on Monday said the event was not likely related to the anniversary. Investigators have found a motive but aren't releasing additional information, said an FBI spokesman.

Moore, who is considered dangerous, was released from federal prison April 13 after serving time for his conviction in the May 2005 robbery of a West Virginia bank, according to the U.S. Bureau of Prisons. Federal court records show that Moore pleaded guilty to robbing the bank in Crab Orchard, W.Va., of $2,546.

A judge sentenced Moore to between 15 and 19 years in prison, but a federal appeals court in 2006 ruled his stiff sentence was "unreasonable" and Moore's sentence was reduced to seven years.

Moore was photographed walking into the Southwest Plaza Mall around noon Wednesday carrying a plastic Target shopping bag, FOX 31 reports.  

Marshall Wood, the employee who found the explosives ablaze in the hallway, described them as camping propane canisters, which are readily available at many stores.

“When I first looked, all I really saw were flames,” Wood told Fox 31. “It looked like there was some melting going on.”

The explosives included two propane canisters and a pipe bomb that were placed in a hallway near the mall’s food court.

The mall, which was evacuated, is a busy indoor shopping center in Jefferson County.

Investigators have exhausted all leads locally and have put out an alert to every FBI office in the country, FBI spokesman Dave Joly said.

"Last time we knew where he was locally was around noon Wednesday at the mall," he said, referring to the previously released surveillance photos. The photos show a man believed to be Moore riding a public bus leaving the mall the night before the fire. Other photos showed the suspect carrying a plastic shopping bag while riding an escalator.

Moore is described as being between 6 feet to 6 feet 2 inches tall and weighing about 200 to 225 pounds. He's bald with blue eyes and a grey mustache and has a tattoo of a Viking on each of his arms.

The FBI says Moore has been known to assume the aliases John Lindzy, Earl Buchanan, Morelli Buchanon, Donald Morelli and Gary Steele, according to Fox 31. 

Click here for more on the attempted Colorado mall bombing from Fox 31 

The Associated Press contributed to this report