Cleanup of an estimated 400-gallon spill of a mix of crude oil and water into the Port of Valdez continued Sunday, three days after the spill was first reported.

The Alyeska Pipeline Service Co. is now trying to determine what exactly caused the release of a mix of oily residue and seawater from pipes during a test Thursday.

"We know what happened. We don't know how or why it happened," said company spokeswoman Kate Dugan from Valdez.

The mechanism for the spill is known, she said: For a test of the system used to load oil into tankers, oil had been drained out of piping and replaced with seawater, which was being drawn in at pressure. When the test was paused, seawater mixed with crude oil residue flowed backward through the pipes, discharging into water at the port.

"Something went wrong during the test," Dugan said.

An internal investigation has been launched to determine why and how that happened, Dugan said. Why the test was paused will be part of that investigation.

The Alaska Department of Environmental Conservation said in a statement that it is working with Alyeska to determine the cause of the spill. The DEC initially said a check valve in the system may have failed, causing the release.

At first, the spill was estimated at less than 100 gallons. Company officials revised that number upward to 200, and then to 400.

Pinpointing the exact volume of the spill has been difficult because the discharged substance is a mix of seawater and oil to begin with, Dugan said.

"If it was pure oil, we would be able to know the volume of the pipe and get an estimate," she said.

Concentrated patches of oil have been removed from the water, Dugan said.

As of Sunday, all that's left is "rainbow sheen," which is highly visible, she said. Cleanup efforts Sunday focused on getting remaining areas of sheen off the water.

No harm to wildlife has been reported by observers, Dugan said.