Airlines pulled dozens of Boeing Max 737s out of service for inspections after the aircraft maker told them about a possible electrical problem, the latest setback for the plane.

Boeing said Friday that the issue affected planes used by 16 airlines. The company did not say how many planes are affected or how long it will take for inspections and, if necessary, repairs.

Max jets were grounded worldwide in March 2019 after two crashes killed 346 people. The planes resumed flying in December after regulators in the U.S., Europe, Canada and Brazil approved changes Boeing made to an automated flight-control system that played a role in the crashes.

Boeing said the new issue, in which a component in the electrical power system might not be correctly grounded, was unrelated to the flight-control system.

Southwest Airlines, which began flying the Max again last month, said it removed 30 of its 58 Max jets from its schedule to inspect them. Southwest spokesman Brian Parrish said the airline has not experienced any known problems related to the electrical issue.

Southwest has a fleet of more than 700 Boeing 737s, most of them older models than the Max. Parish said the airline will use other planes to operate flights that had been scheduled with Max jet, and it anticipates only minimal disruption to its operation.

American Airlines temporarily removed 17 of its 41 Max planes from service, according to a memo to employees. Boeing traced the issue to a production change made after the worldwide fleet of Max jets was grounded in 2019, American's chief operating officer, David Seymour, said in the memo.

“We will never knowingly operate an aircraft with a known or potential issue,” Seymour said.

American was the first U.S. airline to resume flying the Max, in late December.

United Airlines said it temporarily grounded 16 of its 30 Max planes and will try to operate all flights with its other planes. The airline said it was in contact with Boeing and federal regulators to determine what steps are needed before the planes can return to service.

Alaska Airlines said it removed all four of its Max jets from service “to allow for inspections and for work to be done.”

Boeing said the electrical issue was discovered on a plane on the production line. The company said it is working closely with the Federal Aviation Administration on the matter.

Since the Max crashes, Boeing has made internal changes in response to accusations that it short-changed safety in pursuit of profits. Lawyers suing Boeing over the crashes seized on the latest problem with the planes.

“This grounding calls into question the safety of the Max and why Boeing’s claims of a ‘changed culture’ continue to ring hollow," said Steven Marks, one of the lawyers representing relatives of passengers who were on board an Ethiopian Airlines Max that crashed in March 2019. “We don’t know what other issues exist.”

Shares of Boeing Co., based in Chicago, fell about 1% in afternoon trading.