FBI agents in Texas kept tabs on opponents of the Keystone XL pipeline for months before deciding their group wasn't a security threat, the latest case of federal surveillance that has environmentalists feeling hassled by authorities.

The US Federal Bureau of Investigation (FBI) says it was only conducting an "assessment" of a potential threat to existing infrastructure and closed the book on the matter after deciding no threat was posed. But that's little comfort for advocates who say the top US law enforcement agency is wasting its time snooping on peaceful protesters.

"This is really business as usual for the FBI," Will Potter, whose book Green is the New Red documents federal probes into environmental groups, told VICE News.

'At this point, there's been a repeated pattern of our government siding with a big oil company against people concerned about the effects big oil has on their lives.'

Members of the Houston-based Tar Sands Blockade have chained themselves to construction equipment in protests aimed at stopping work on the southern end of the pipeline, which would carry heavy crude extracted from the tar sands of western Canada to refineries in coastal Texas. The Obama administration is still weighing whether to approve the project's northern end, which would speed one of the heaviest and most carbon-intensive sources of fuel to market.

Using documents obtained under the US Freedom of Information Act, the British newspaper The Guardian and the US-based Earth Island Journal reported this week that the FBI's Houston field office had spied on the Texas activists as part of its assessment. An FBI memo called the pipeline project "vital to the security and economy of the United States" and warned that "environmental extremists" were gearing up to oppose it.

Ron Seifert, a spokesman for Tar Sands Blockade, said the group held dozens of demonstrations in Texas and Oklahoma during that time, with about 130 people arrested for trying to block pipeline work. Seifert said the documents are heavily redacted, but indicated at least one person in the group discussed its actions with the FBI.

"We were running such a public direct action campaign we could only assume we would attract attention of the state in that regard," he said. But the FBI documents read like they're "straight off the script" of TransCanada, which he said paint pipeline opponents as "dangerous and violent and threatening."

"Of all of those 130-plus arrests, not a single charge anywhere along the line has anything to do with property destruction, anything to do with violence or assault or anything like that," Seifert said. "They were all nonviolent charges, simple things like trespassing by and large."

FBI rules allow agents to use publicly available information, government records, interviews, and "observation or surveillance not requiring a court order" as part of an assessment, a kind of preliminary inquiry.

The monitoring stretched from late 2013 into mid-2014, according to the documents cited by the publications. And it apparently ran afoul of the bureau's own rules by not being approved by the top agent or lawyer in Houston, though the bureau told VICE News that those officials signed off later.

"While the FBI approval levels required by internal policy were not initially obtained, once discovered, corrective action was taken, non-compliance was remedied, and the oversight was properly reported though the FBI's internal oversight mechanism," the bureau said in a written statement. "At no time did the review find that the initial justification for the assessment was improper."

Once the error was found, the top officials in Houston approved the assessment, which continued "under full compliance" of the Justice Department's guidelines, the FBI statement said. Oil and gas installations are "part of the critical infrastructure of the United States" and may be the targets of terrorists or spies, it added.

But it's at least the second time this year opponents of the proposed pipeline were the objects of FBI inquiries. Anti-Keystone campaigners in the Northwest reported getting visits from agents who wanted to learn more about a radical group that has called for sabotaging infrastructure, Toronto, Canada's The Globe and Mail reported in February.

And in January, an eco-activist who spent nearly nine years behind bars was released after his lawyers uncovered thousands of documents that showed the FBI withheld evidence in his trial. Eric McDavid had been convicted in a plot to bomb a California dam, based mostly on the testimony of an FBI informant who infiltrated his group and urged it to adopt more aggressive tactics.

Tar Sands Blockade did not return messages seeking comment. But Greenpeace researcher Connor Gibson told VICE News that spying by both government agencies and corporations "is something the environmental community has dealt with for a long time."

Greenpeace has been targeted by governments and corporations alike since its founding in 1971. But Gibson said the Texas case is interesting "because there's such an overlap between corporate and government spying." The Guardian reported that FBI officials met with TransCanada several months before the Houston office opened a file on Tar Sands Blockade.

"At this point, there's been a repeated pattern of our government siding with a big oil company against people concerned about the effects big oil has on their lives," he said.

Potter said the FBI's interest in environmental activists has undergone a "seismic shift" since the 1990s, when movement radicals claimed responsibility for the burning of a ski lodge in Vail, Colorado and fires and sabotage at car dealerships and construction sites in other states. Even after the September 11, 2001 attacks on New York and Washington, when much of the bureau's resources shifted toward fighting terrorism, the bureau has kept an eye on the greens.

"At first, the assessment investigations were justified based on the specter of causing a loss of human life, that eco-terrorists were somehow going to kill innocent people," Potter told VICE News. "That's never happened. Then the justification became more and more that the FBI was investigating potential property destruction, and increasingly that doesn't happen either."

Now, with environmentalism much more mainstream, the FBI says it's trying to prevent economic damage. But activists "aren't threatening lives and property, they're threatening money," Potter said.

In 2003, an inspector-general's report urged the bureau to leave probes of environmental and animal-rights activists to its criminal division. But Potter said the FBI never made the recommended changes, even when it was urged to pursue "more credible and dangerous" threats after 9/11.

"What started as a corporate-driven agenda to label protesters as eco-terrorists has become institutionalized," he said. "This has really become standard operating procedure, and I think that's what's most disturbing about this."