SALT LAKE CITY — The U.S. Bureau of Land Management is warning its workers in Utah to be on alert after two men threatened an agency wrangler on the state's main highway by pulling out a weapon and holding up a sign that read, "'You need to die."

The incident occurred Tuesday morning on Interstate 15 about 90 miles south of Salt Lake City, said BLM spokeswoman Megan Crandall. A BLM employee was driving his agency vehicle and pulling a trailer when two hooded men came up alongside him in a dark blue Dodge truck and flipped him off, Utah Highway Patrol Sgt. Corey Houskeeper said.

The truck slowed down before the men came up beside the BLM officer again and pulled out the sign and flashed a firearm, he said. No one was hurt, as the men sped away.

An investigation is underway, but there are no suspects. The BLM officer wasn't able to get the license plate number because it was covered by what appeared to be duct tape, Crandall said.

Highway troopers searched the area for the truck, and the agency has put out an alert statewide for the truck, but Houskeeper said it will be difficult to find the men since there could be thousands of trucks that fit the description.

This is the first incident of this type in recent years, Crandall and Houskeeper said. But it comes during a time of high tension between some Western residents and the Bureau of Land Management.

Last month, the BLM stopped rounding up Nevada rancher Cliven Bundy's cattle after a showdown with hundreds of Bundy supporters, some of them armed. Bundy, a states' rights advocate who refuses to acknowledge the federal authority, owes more than $1 million in fees and penalties for letting his cattle use government land over the past 20 years.

More recently, rural Utah ranchers and county leaders threatened to break federal law and round up wild horses this summer if the BLM doesn't do it first. State wildlife officials voted to back the ranchers.

On Saturday in a southeastern Utah canyon, San Juan County Commissioner Phil Lyman plans to lead ATV riders on BLM land in an area where motorized vehicles are banned. Lyman is using the demonstration to show his displeasure with the federal government's closure of the trails.

The federal government owns two-thirds of Utah's land, and a debate about what activities should be allowed on that land has been ongoing. The Republican-dominated Utah Legislature passed a law in 2012 that calls on the federal government to hand over control of all public lands before 2015, excluding national parks.

The BLM issued a statement that said, "Threats against Bureau of Land Management employees will not be tolerated, and we are pursuing this matter with local law enforcement."

The BLM's state director, Juan Palma, said Thursday that he remains calm despite the serious incident.

"We always are worried about our employees, but I believe that most citizens here in Utah are very law abiding," Palma said.

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Associated Press writer Michelle L. Price contributed to this report.