An initial examination of the Pratt & Whitney PW4077 engine from that flight showed that two fan blades were fractured, according to the US National Transportation Safety Board (NTSB). The agency said that the remaining blades exhibited damage "to the tips and leading edges."

The findings are preliminary, but they were still enough to spur some airlines to pull planes that use those types of engines from service. The NTSB said that it will examine the engine, airline and photos and videos taken by passengers on board the flight, along with the flight data and cockpit voice recorders.

Investigators believe a fan blade in the engine came off and took out another blade, a source familiar with the situation told CNN.

Grounded flights

Japan's transportation ministry, meanwhile, said that it had ordered All Nippon Airways and Japan Airlines to ground planes using that series of engines. The ministry said that order applied to 19 Boeing 777s at All Nippon Airways and 13 at Japan Airlines that have Prat & Whitney 4000 engines.

South Korea's Asiana Airlines, which owns nine of the planes, said it would stop using them, too.

The jets are also used by Korean Air. That airline said Monday that it would hold off on making any decision until the country's transport ministry issued guidance.

According to the most recent registry data, the only airlines that operate with the affected engines are in the United States, Japan and South Korea. United is the only US operator with this type of engine in its fleet.

Ongoing investigation

Both Boeing and Pratt & Whitney have said they are cooperating with the US investigation into the Denver incident.

"We are pleased to hear that United Airlines flight 328 returned safely to Denver," Boeing said in a statement. "Boeing technical advisers are supporting the US National Transportation Safety Board with its investigation."

"Pratt & Whitney is actively coordinating with operators and regulators to support the revised inspection interval" of the 4000 engines that power certain 777s," the company said, adding that "any further investigative updates regarding this event will be at the discretion of the NTSB."

The US Federal Aviation Administration on Sunday also issued an emergency order saying it would be stepping up inspections of 777s that use certain Pratt & Whitney 4000 engines.

"We reviewed all available safety data following [the] incident," FAA Administrator Steve Dickson said. "Based on the initial information, we concluded that the inspection interval should be stepped up for the hollow fan blades that are unique to this model of engine, used solely on Boeing 777 airplanes."

— Gawon Bae, Sophie Jeong and Junko Ogura contributed to this report.