Arkansas Attorney General Dustin McDaniel said Tuesday that his office has opened an investigation of a crude oil spill near Mayflower, AR, which ExxonMobil was working to clean up. The cause of the spill is still under investigation, but Exxon said the spill did not involve tar sands oil.

McDaniel also said he has directed the company to “preserve and maintain” all documents related to what he called an “extensive” oil spill occurring last Friday. The state official has committed to investigating the cause and impact of the rupture of ExxonMobil’s 850-mile Pegasus pipeline, which at times carries tar sands oil from Canada, although it wasn’t at the time of Friday’s spill.

The head of the Arkansas Sierra Club, Glen Hooks, called the spill “a tragic warning” regarding the proposed northern portion of the Keystone XL $7 billion proposed oil pipeline that involves tar sands-produced supplies. Hooks said he could see “thick viscous tar sands oil” in residential neighborhoods where people had been evacuated.

In its latest update, ExxonMobil said on Tuesday a plan was being developed to return residents on a phased basis to 22 evacuated homes in a subdivision of Mayflower. An Exxon spokesperson told NGI’s Shale Daily at the time of the spill the Pegasus pipeline was carrying heavy crude from Canada, but not tar sands supplies. At other times, he confirmed, it does carry tar sands oil.

An industry source noted that ExxonMobil may be splitting hairs in saying that the heavy crude flowing through the Pegasus pipeline was not tar sands-produced oil because there is only a technical difference between the two. They are both heavy in viscosity and require help in making them more transportable.

Exxon’s website description of the Pegasus line describes it as “a 20-inch [diameter] pipeline which originates in Patoka, IL and carries crude oil to the Texas Gulf Coast.”

“Thousands of gallons of crude oil leaked into a residential neighborhood near Lake Conway, leaving significant damage to the state’s environment and to property in the surrounding area,” McDaniel said.

ExxonMobil said nearby Lake Conway was not impacted, and the unified command response has a containment system in place, using a boom laid out as a precautionary measure.

As an excavation removal plan for a portion of the pipeline is being developed with the U.S. Department of Transportation for its investigation, ExxonMobil said a clean-up plan was underway, including establishing a claims hotline for residents affected by the spill, (800) 876-9291. On Monday, the company said about 90 claims had been submitted so far.

The operator confirmed that under federal Environmental Protection Agency criteria this was a “major spill,” reflecting quantities greater than 250 bbl. “A few thousand barrels of oil were observed in the area, [and] a response of 10,000 bbl has been undertaken to ensure adequate resources are in place,” an ExxonMobil Pipeline Co. spokesperson said.

The pipeline operator said Tuesday that 12,000 bbl of oil and water have been recovered since the Pegasus pipeline started leaking. Nearly two dozen homes in the area, located about 20 miles northwest of Little Rock, were evacuated.

“This incident has damaged private property and Arkansas’ natural resources,” McDaniel said. “Requesting that Exxon secure these documents and data is the first step in determining what happened and preserving evidence for any future litigation.”

Specifically, the attorney general’s letter asked the oil giant’s pipeline to require any affected employees and affiliated organizations to preserve all “documents, data compilations (including electronically recorded and stored data), tangible objects or other information” relevant to the pipe rupture.

ExxonMobil in a report from Mayflower on Monday said it had 15 vacuum trucks and 33 storage tanks deployed to the site as part of the clean-up plan. There are approximately 120 Exxon pipeline unit employees responding to the incident in addition to various federal, state and local resources.

A community meeting was held by the company Saturday in the immediate wake of the incident last Friday. Company, state and local officials made presentations. Exxon said emergency response workers were on the ground within 30 minutes of the rupture.