COLORADO SPRINGS, Colo. -- Hikers in the area of Seven Falls over the weekend stumbled onto a stolen bag of ammonium nitrate-based explosive, the same type used in the 1995 Oklahoma City bombing.

The 50-pound bag was one of eight stolen last summer from a concrete company in northwest Colorado Springs. There was no sign of the other bags.

"Three-hundred fifty pounds are still out there somewhere," said Rick McMorran, a bomb technician from the El Paso County Sheriff's Office.

The amount of ammonium nitrate and fuel oil mixture stolen was enough to blow apart a three-story building, authorities said. A 50-pound bag would be enough to destroy an average-size house, the sheriff's office said.

Searchers with a bomb-sniffing dog on Monday went to the spot where hikers found the bag -- on Mesa Avenue near the entrance of Seven Falls Park -- but turned up nothing.

The material was stolen in July from a locked storage unit at Castle Concrete Co. at the Pike View Quarry in northwest Colorado Springs. The ammonium nitrate and fuel oil (ANFO) is a blasting agent commonly used in quarry and mining operations.

ANFO can't be detonated without several other components.

Authorities are still searching for the missing seven bags. The bags are brown in color, with the words "Buckley Powder Co." and "ANFO" followed by an orange diamond shaped label. The explosive itself is made up of small white balls like those found in lawn fertilizer, and it has a diesel odor.

The nitrate becomes more volatile as it is crushed into a finer powder.

An estimated 4,800 pounds of ANFO were used for the bomb that blew up the Alfred P. Murrah Federal Building in Oklahoma City and killed 168 people. Denver juries convicted Timothy McVeigh and Terry Nichols on federal charges in the bombing.

McVeigh was executed. Nichols was sentenced to life in prison and is now being tried on state charges in Oklahoma.

ANFO is also the main explosive used in the first attack on the World Trade Center in New York City.

Anyone having information on the location of the remaining bags is asked to call the sheriff's office, police department, or Crime Stoppers at 634-STOP. Callers can remain anonymous and earn up to $1000.