ANCHORAGE, Alaska (AP) — The cause of a natural gas leak from a line that provides fuel for two Hilcorp Alaska offshore production platforms in Cook Inlet is being investigated, the Alaska Department of Environmental Conservation said.

The department released an initial update on the leak Monday, four days after it says a helicopter pilot spotted bubbles on the water’s surface. The leak was reported to authorities, including the department, by Hilcorp about an hour later that same day, April 1, the report states.

Hilcorp shut down the platforms in response, the report stated. The company said the leak was stopped on Saturday by activating block valves.

Hilcorp spokesperson Luke Miller in a statement Tuesday said the company is monitoring ice conditions and that sonar scans were planned to gather data.

“Divers will be deployed mid-week to install a temporary clamp,” he said.

Miller said no personnel or wildlife “have been impacted.”

The environmental conservation department in its Monday report said the size of the leak was unknown.

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Anna Carey, an environmental program specialist with the department, said by email Tuesday that Hilcorp will work with the federal Pipeline and Hazardous Materials Safety Administration to investigate the cause.

Carey said the line is regulated by the federal pipeline agency and that the state Department of Environmental Conservation won’t play a role in the investigative process.

The Anchorage Daily News reported that a leak also occurred from the same pipe in 2017. In that case, the leak lasted for months and led to requirements for closer inspections by Hilcorp to prevent future leaks.

The line, installed in Cook Inlet in the 1960s, originally carried crude oil before it was converted to ship natural gas as fuel. It extends about 7 miles (11 kilometers) from the coast to reach the platforms, the newspaper reported.

Hilcorp cited abrasion from an underwater boulder for the 2017 leak. Rock abrasion and strong inlet tides caused leaks on another section of the line in 2014, before XTO Energy sold it to Hilcorp.

The 2017 leak was sealed by a temporary clamp. But Carey told the newspaper that leak went on longer partly because Hilcorp was concerned that minimal power was needed for the platforms, a concern they overcame this time.

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Hilcorp also worried then about depressurizing the gas line too much and possibly allowing residual oil to escape from the line, Carey said.

In the current case, Hilcorp worked with the federal pipeline agency to depressurize the line before the leak was stopped, Carey said.

Hilcorp is the leading oil and gas producer in Cook Inlet.