Judicial Watch announced on Wednesday it filed suit against the Department of Defense and Army Corps of Engineers, seeking to obtain communications between government officials in the Obama administration and environmental groups related to protests last year opposing the Dakota Access pipeline.

The conservative watchdog group's Freedom of Information Act lawsuit, filed June 29 in the U.S. District Court for the District of Columbia, charges that the Obama administration and environmentalists worked "hand-in-glove" to try to stop the pipeline's construction and asks to review communications between the two camps to prove it.

Judicial Watch President Tom Fitton said his group issued a FOIA with the Defense Department for the communications in May but has not received a response. The group wants communications between the Army Corps of Engineers, which had to review and sign off on the project, and Greenpeace, Sierra Club, EarthJustice and Friends of the Earth.

"Barack Obama and radical – and often violent – environmentalists worked in hand-in-glove to shut down the Dakota Access pipeline. We're not sure why the Trump Defense Department would hide the facts about this scandal and force us to go to federal court to enforce FOIA," Fitton said in a statement.

The suit relates to months of protests last year over the 1,172-mile pipeline, built to deliver shale oil from North Dakota to Midwestern refineries.

The Standing Rock Sioux tribe and other Native American groups argued that a stretch of the pipeline, across the Missouri River in North Dakota near the Standing Rock Sioux's reservation, would taint drinking water if it leaked and disturb sacred burial and archaeological sites.

In December, the Army Corps under the outgoing Obama administration ceded to the tribes' concerns and blocked the disputed section of the project.

Trump signed an executive order directing the Army Corps to move forward with the project soon after being sworn in as president. But in June, District Court Judge James Boasberg responded to lawsuits by ruling that the government must reconsider parts of the environmental review that deal with the lake segment.

The pipeline began shipping oil in June.

The developer of the pipeline, Energy Transfer, filed its own federal lawsuit on Tuesday against Greenpeace and other environmental groups, claiming they encouraged Native American groups to engage in protests against the pipeline, increasing the cost of construction by at least $300 million.