The move underscores mounting tensions between the state and one of the world's largest energy companies over its handling of pipeline rupture that spewed tens of thousands of gallons of oil into the scenic river.
Gov. Brian Schweitzer was to travel to Billings Friday to announce the opening of an alternate state-run oil spill coordination center.
Exxon Mobil security workers have closely guarded access to the command post on the second floor of the Crowne Plaza Hotel in Billings, where the EPA and other federal agencies also are stationed. Attempts by The Associated Press to talk to government officials there have been denied.
"The state will no longer have a presence at the Crowne Plaza because Exxon Mobil tells us they can't respect the open government laws we have in Montana," Schweitzer told The Associated Press. "I can't allow state employees to be in meetings at the Crowne Plaza talking about this cleanup without having it open."
Schweitzer said the move would not impede the state's ability to respond to the spill.
Exxon Mobil spokesman Alan Jeffers said the company was not in charge of the command post, a joint operation led by the U.S. Environmental Protection Agency.
"We do not run the unified command. We are providing security services for the unified command, just like we are providing cleanup serves for the unified command," he said.
EPA spokesman Matthew Allen said in an email to the AP that the agency was still directing the cleanup and would continue "to work hand-in-hand with the state of Montana, other federal agencies, and local government to ensure the spill is cleaned up and the environment restored."
"We've committed to the governor and the people of Montana that we're staying until the job is done and we stand by that commitment," Allen said.
Indoor air, cropland soils and residential wells downstream of the July 1 oil spill will be tested for contamination after residents raised concerns about hazards from the tens of thousands of gallons of crude that poured into the watercourse, the Environmental Protection Agency said.
EPA and local officials said they do not expect to find significant health dangers but were acting as a precaution. Some residents in oil-stained areas have complained of nausea, dizziness and shortness of breath that have lingered for days.
An estimated 1,000 barrels of oil, or 42,000 gallons, have fouled areas along the scenic Yellowstone since Friday after a 12-inch pipeline operated by Exxon Mobil Corp. broke near the south-central Montana town of Laurel.
George Nilson, 69, of Billings, said the fumes from oil that washed into his neighbor's property have been overwhelming.
"I've been in it for five days now, and the only way I can breathe is to have all the windows open," he said.
Contractors for the EPA and Exxon Mobil were to collect air samples beginning Thursday or Friday, and the results would take about a week, said EPA on-scene coordinator Steve Merritt. Twelve homes would be tested initially, with possibly more to follow.
Crude oil contains dangerous chemicals including benzene and hydrogen sulfide. But officials said much of those substances would have evaporated quickly after the initial spill, meaning the long-term health risk is low.
Air sampling along the river has not detected either of the chemicals, and water sampling shows no petroleum hydrocarbons that exceed drinking water standards, the EPA said in a written statement late Thursday.
"The air is not inundated with these potentially harmful chemicals," said Yellowstone County Health Officer John Felton. "We can smell things that are no longer creating the same level of hazard."
Soil from agricultural areas and water from hundreds or residential wells also will be tested in coming days, Merritt said. Exxon Mobil's contractors will collect duplicate samples so their results can be verified by government scientists, he said.
Authorities in Yellowstone County said they would ease travel restrictions along a road near the spill site after some area residents and members of the media complained about a lack of access.
Those restrictions at times have been enforced by private security contractors working for Exxon Mobil, who turned away reporters or blocked them from areas where cleanup work was going on.
"We have been frustrated since the spill took place because we've burned up time waiting for Exxon officials or other authorities to respond to our request for information and access," said Steve Prosinski, editor of the Billings Gazette. "We realize cleanup is their primary focus but they have a responsibility through us to communicate how the cleanup is going."
Yellowstone County Sheriff Mike Linder said his deputies were working in conjunction with the company but had not ceded any authority to it. Linder said the restrictions were meant to protect public safety.
"They're not calling the shots down there as far as access," Linder said of Exxon Mobil. "They'll let us know when there is a safe time or not a safe time. We're working together, is what we're doing. If it's a safety issue, we will address it. If it's not, we will work with them to make sure everybody has access."
Jeffers said the company was trying to be transparent and has worked over the week to improve media access to cleanup areas.
Federal regulators have ordered Exxon Mobil to make safety improvements before re-starting the 20-year-old pipeline, including re-burying the line as much as 25 to 30 feet deep to protect against external damage and assess risk where it crosses a waterway.
There is still no definitive word on how far downriver the spill could spread.
There have been confirmed reports of oil as far as 80 miles downstream, although most is concentrated in the first 30 miles, according to the EPA. Allen said the agency didn't expect to find much more oil beyond the 80 mile mark, aside from "small, isolated quantities."
An estimated 350 federal and Exxon Mobil contractors were cleaning contaminated areas of riverbank by Thursday, said Exxon Mobil Pipeline Co. President Gary Pruessing.
"It's not soiled everywhere but there are pockets of it," Pruessing said. "It's going to take a while as we try to get our hands around where the contamination is and then clean it up."
The cause of the July 1 pipeline rupture remains under investigation.
The Department of Transportation had stated in documents released earlier this week that the company reported on June 1 that the line was buried under "at least 12 feet of cover" where it crosses the river near Laurel.
A DOT spokesman on Thursday clarified the 12-foot figure as applying to the section of pipeline beneath the bank of the river.
The depth of the section beneath the central portion of the riverbed was measured by the company in December at 5 to 8 feet. Determining its depth when the pipe failed will be part of the federal investigation into the spill.
The depth was measured after officials in Laurel raised concerns about the safety of the pipe because of erosion along that stretch of the river. In 2009, a Williston Basin Interstate Pipeline Co. natural gas line that crossed in almost the same spot ruptured during high waters.
