Prolonged exposure to the substance found inside the rocks has been shown to cause lung cancer, and investigators recommended in a draft report that signs be posted "alerting people to the dangers of asbestos exposure."
Residents, however, were never formally notified of the discovery by federal, state or local officials a case that experts and others say highlights the challenges authorities face when dealing with naturally occurring hazards.
Jean Melious, an environmental and land-use lawyer who teaches environmental studies at Western Washington University, said natural asbestos is vexing for government agencies, partly because it's not a disaster that calls for immediate action.
"It's a lot easier for government to work when there's a big hue and cry," she said.
Andy Smith, an on-scene coordinate with the EPA, said no government agency has total authority over natural asbestos. "This kind of problem falls in the cracks," he said.
Natural asbestos is often found in certain types of rocks and near fault zones. It can be released into the air from the rocks when they are broken or crushed, as often occurs during mining or development.
The Washington Department of Natural Resources says naturally occurring asbestos has been found in areas in the northern part of the state and experts say it occurs throughout the United States.
Keith Welch, a resident of the Burlington Hill housing development who alerted federal authorities, said he's frustrated the authorities haven't been more proactive in telling people about the presence of asbestos and doing more comprehensive studies.
"It's one thing to be cautious," Welch said. "Now that it's been identified, somebody needs to do something about it."
The Burlington Hill case has helped agencies discuss best practices for permitting and public awareness, said Katie Skipper of the Northwest Clean Air Agency, which is responsible for enforcing air pollution regulations in Skagit, Whatcom and Island counties.
In June, Skagit County posted information about naturally occurring asbestos in the environmental health section of its website. It mentions the asbestos on Burlington Hill in one sentence, and provides links to other asbestos-related information.
Polly Dubbel, a Skagit County environmental health specialist, said residents weren't notified and most already likely knew of the presence of asbestos because of a website Welch created to publicize a lawsuit he filed against the city of Burlington.
Welch, a developer who has also built homes on Burlington Hill, said he contacted the EPA after he learned there was an old asbestos quarry in the area. Welch had sued Burlington in 2008 in a dispute over a road rebuilding project on Burlington Hill.
He amended his lawsuit last year after the September 2012 EPA investigation, saying the roadwork potentially exposed dozens of people who live in the area to the asbestos.
In its March 2013 final report, the EPA said it found actinolite asbestos along a road cut on the northeastern side of Burlington Hill. No asbestos was found at three other locations sampled.
Because of the health risks associated with asbestos, the report said people should limit their exposure to the asbestos and that a more thorough study would need to be done to determine how much asbestos might be at the site.
In a draft of the final report, obtained by The Associated Press via a public disclosure request, EPA investigators recommended Skagit County post warning signs.
Smith, one of the EPA staffers who went to Burlington Hill, said the EPA didn't include the recommendation on the warning signs in the final report because that went beyond the agency's mandate.
"Our place is really to find out if there was any big, screaming source for us to clean up. And there wasn't," Smith said.
He added, "Another complicating part is we have no statutory authority to clean up naturally occurring asbestos."
EPA investigators collected samples for testing by breaking off bits of exposed rock with a hammer.
In their final report, the investigators said given the limited nature of the study at Burlington Hill, they couldn't say what risks people exposed to the asbestos could face.
For that, air samples that measure asbestos concentrations that people could breathe would be needed. But the investigators said, "EPA would caution people to refrain from disturbing the material" where the asbestos was found.
Joanne Snarski from the state Department of Health said state and local authorities can help developers and landowners be aware of areas where things like naturally occurring asbestos and other natural hazards can occur and work to mitigate hazards.
"There's a variety of naturally occurring issues that people live with on a regular basis," she said, adding there's a balancing act between public awareness and possibly overstating potential risks.
Both Smith and Snarski said issuing specific directives to landowners can be difficult. "A lot of people are pretty uncomfortable with people telling you all the things you can and cannot do on their property," Snarski said.