Attorney General Dustin McDaniel speaks at a news conference Thursday beside U.S. Attorney for the Eastern District of Arkansas Chris Thyer. ( Gavin Lesnick

The state of Arkansas and the United States have filed a joint complaint against Exxon Mobil over a burst pipeline that spilled thousands of barrels of oil into a Mayflower neighborhood, displacing residents from 22 homes and raising environmental concerns, officials announced Thursday.

Arkansas Attorney General Dustin McDaniel said at a news conference that the lawsuit was filed in U.S. District Court (PDF).

"The oil spill disrupted lives," McDaniel said. "This oil spill harmed the environment. And this oil spill was in violation of both state and federal law."

McDaniel spoke at the Arkansas Department of Environmental Quality's North Little Rock headquarters, joining that agency's director, Teresa Marks, and U.S. Attorney for the Eastern District of Arkansas Chris Thyer for the formal announcement of the complaint.

The complaint seeks civil penalties for violations of the Arkansas Water and Air Pollution Control Act and the state Hazardous Waste Management Act. It also alleges that Exxon illegally stored waste from the cleanup at a site on Arkansas 36 in Conway.

McDaniel said he would not "speculate" on the total amount of penalties sought in the lawsuit because the investigation is continuing and the figure could change.

State law provides for penalties of $10,000 per violation per day for the Water and Air Pollution Control Act and $25,000 per violation per day for violations of the Hazardous Waste Management Act. The federal Clean Water Act also allows penalties between $1,100 and $4,300 per spilled barrel.

The higher penalty would be allowed if gross negligence or willful misconduct is proven. An ongoing investigation will reveal whether those standards are met, Thyer said.

"We don't have those facts yet," he said. "We don't know those facts."

McDaniel said the illegal storage in Conway included contaminated soil, oil and water mixtures and debris. And he said the company failed to move the complete operation even after being told last month an ADEQ permit they didn't have was required.

"To store this material without following the law in advance and then to not remove it upon being demanded to do so, we felt was particularly concerning," McDaniel said.

The Pegasus pipeline ruptured March 29 in the Northwoods subdivision, seeping an estimated 147,000 gallons of crude oil into the neighborhood. The oil spread through the streets and into drainage ditches, eventually reaching a cove of Lake Conway about a mile away.

A joint cleanup effort followed with representatives from the oil giant, Faulkner County, the state and the federal government working to sop up the spilled oil and attend to affected wildlife in the area. Officials have said the oil was prevented from ever reaching the main body of water in Lake Conway.

McDaniel said Thursday the spill has had a "significant and lasting negative impact on our state's environment."

"And Exxon, as the responsible party for the incident, should be penalized for those impacts," he said.

Cleanup efforts have been ongoing in Mayflower, and some residents evacuated from their home were cleared several weeks ago to return. But as of Thursday, none had opted to do so. Exxon Mobil has said it will purchase the homes at pre-spill appraisal prices. Several lawsuits have already been filed.

The cause of the spill has not been determined, McDaniel and Thyer said at the media briefing. Exxon Mobil recently received an extension until July 10 to provide to the Pipeline and Hazardous Materials Safety Administration the results of mechanical and metallurgical testing on the portion of the pipeline that cracked. That analysis is expected to reveal the cause.

See Friday's Arkansas Democrat-Gazette for more on this story.