Investigators combing through an Oregon wildlife preserve occupied by an armed group for nearly six weeks have so far discovered firearms, explosives and trenches dug near an area containing tribal artifacts, according to federal prosecutors.

The FBI also was concerned that numerous vehicles found at Malheur National Wildlife Refuge might be booby-trapped, Assistant U.S. Attorneys Ethan Knight and Geoffrey Barrow said in a court filing Tuesday.

"Occupiers appear to have excavated two large trenches and an improvised road on or adjacent to grounds containing sensitive artifacts," prosecutors wrote. The refuge contains artifacts and burial grounds sacred to the Burns Paiute Tribe.

Investigators also found human feces in one of the trenches and spoiled food in the living quarters.

The FBI expects to take three weeks to process the nature preserve seized Jan. 2 by occupiers demanding the government relinquish control of public lands. More than two dozen people have been charged in the standoff, which petered out after its leaders were arrested Jan. 26 in a traffic stop that also left an Arizona rancher dead.

Four holdouts remained until last Thursday, when they surrendered and investigators were able to move in to search for evidence.

Prosecutors provided the information in response to a motion from defense lawyers who want immediate access to the site. The government is willing to let the defense and its investigators examine the refuge after evidence has been collected but before it reopens to the public, Knight and Barrow said.

They said the defense teams must be escorted by FBI personnel and cannot disturb or remove items. All evidence seized by the FBI will be available for inspection at the agency's field office in Portland.

Defense lawyers are challenging that arrangement, and a judge is expected to hear arguments on the matter next week.

Attorneys for jailed standoff leader Ammon Bundy said they were concerned about damage that might have been done by the final occupiers after Bundy was arrested.

"The government will not know what evidence at the scene is specifically favorable to Mr. Bundy as opposed to another co-defendant," Lissa Casey and Mike Arnold wrote in response to the prosecutors' filing. "Many different people took many different actions at that scene."