Response crews at a BP well in Alaska’s North Slope that began spouting crude oil and methane last week stopped the leak early Monday morning. Now environmental groups are starting their push for answers.

“A big issue now that it’s under control is making sure there’s a rigorous investigation about what went wrong,” said Lois Epstein, who heads the engineer and Arctic program for The Wilderness Society. “That includes looking into whether this is something that could be more pervasive.”

BP workers spotted crude oil spraying from the top of the onshore well Friday morning, in the remote Prudhoe Bay region of the Alaskan Arctic, but were able to stop the spraying by closing a safety valve. A second leak of methane, a greenhouse gas, continued venting uncontrolled gas into the air through most of the weekend.

“Last night the Unified Command killed the well and stopped the oil and gas leak,” said BP spokeswoman Dawn Patience in an email Monday. “The response operations will continue.”

According to the latest report, released Monday from the Alaska Department of Environmental Conservation, an infrared flight indicated that the oil did not leave the gravel pad surrounding the well, but “cleanup responders have not yet confirmed there are no impacts to adjacent tundra.”

The state had not determined an estimate for the amount of oil leaked or a cause for the mishap.

The accident occurred as the Trump administration is readying an executive order to reverse the Obama administration’s ban on offshore oil drilling in more than 100 million acres in the Arctic and nearly 4 million acres in the north and mid-Atlantic. Halting an oil or gas spill is always difficult, but responders to an accident in the Arctic Ocean would face immeasurable challenges and many unknowns posed by persistent darkness, remote locations and sea ice.

Epstein, an engineer, said that spills in Alaska’s remote North Slope are common, but leaks of this kind are not.

The Department of Environmental Conservation report said that the well “jacked up” during the event, but has since settled by 11 inches. What’s unclear, Epstein said, is what triggered that. The cause could be anything from a maintenance problem to the remote possibility that the ground around the well has been impacted by natural gas injections.

“We’ve been injecting a lot of gas on the North Slope,” Epstein said. “We won’t know if this is going to happen a lot more until we learn the results of the investigation. We won’t know if this is a new hazard.”

The last time a methane leak occurred in the area was in 2012, Epstein said.

According to the state’s report, responders attempted to bring the well under control on Saturday, but were unable to because a broken pressure gauge prevented them from pumping fluids into the well to kill it. On Sunday, a well control contractor was able to plug above-ground piping, enabling them to pump a liquid solution into the well.

The leak is the latest example of infrastructure problems in the state’s gas and oil industry. Last week, Hilcorp Alaska announced it had stopped a methane leak that had been spewing into the state’s Cook Inlet for at least four months.

Hilcorp, one of the biggest privately owned gas companies in the world, buys older gas and oil infrastructure. Most of it, including the natural gas line where the leak occurred, is more than a half century old.

The BP well was built in 1976, shortly after oil development in the North Slope took off. The area produced 2 million barrels of oil at its peak in the 1980s, according to Bloomberg, but now produces about a quarter of that.

Correction: An earlier version of this story mischaracterized the incident at BP’s well in Prudhoe Bay as a blowout. A blowout is a massive release when pressure from the well is released unrestricted. This was a leak, where there was a reduction in pressure followed by a spraying of oil and gas.