The FBI on Monday launched a two-day search through dense and rugged terrain in Washington state in the hopes of finding clues to the whereabouts of Travis Decker nearly three months after he allegedly killed his three young daughters.Investigators still have no evidence about whether Decker is alive or dead, Chelan County Sheriff Mike Morrison and Peter Orth, the FBI's supervisory senior resident agent in Yakima, said during a news conference Monday."You can't be too thorough in a search like this," Orth said. "It is such incredibly dense vegetation that anybody who walks down one of these trails could walk 10 meters off the trail and no one would ever know they're there."Decker, a former and trained military survivalist, has been wanted since June 2, when the bodies of his three daughters Paityn, 9, Evelyn, 8, and Olivia, 5 were discovered at the Rock Island Campground in the Okanogan-Wenatchee National Forest.NEW CRIME SCENE DETAILS EMERGE IN MANHUNT FOR FUGITIVE DAD ACCUSED OF KILLING 3 DAUGHTERSA deputy found the bodies of the girls inside a truck with their hands bound and plastic bags over their heads three days after their mother reported them missing when Decker failed to return the children from a court-mandated custody visit, authorities have said."We will not relent. We will not give up," Morrison said Monday, adding that "the girls would not want us to give up."Authorities closed popular campgrounds and trailheads in the forest near Leavenworth to allow about 100 FBI agents and other law enforcement officers to bushwhack through the rugged terrain.Divers planned to search Icicle Creek to reach areas where logjams had previously barred dive teams, while other investigators conducted a grid search in a quarter-mile radius around the campground, officials said.TRAIL GROWS COLD IN MANHUNT FOR SURVIVALIST FATHER AS AUTHORITIES REACH BREAKING POINT: 'I'M STILL ANGRY'Morrison said that if Decker is still alive in the wilderness, he believes he must be "pretty tired by now.""He's got to be perfect every single day; we just have to be perfect once," Morrison said.Authorities confirmed last week that Decker is the sole suspect in the alleged murders, saying that his DNA was found at the crime scene. No additional DNA profiles were detected."If it's not me wearing this uniform, it'll be another Chelan County Sheriff We will find him, in one fashion or another," Morrison said.The U.S. Marshals Service is offering a reward of up to $20,000 for information leading to Deckers capture.Fox News Digitals Julia Bonavita and The Associated Press contributed to this report.