LOUISVILLE, Ky. (AP) -- UPS and FedEx will ground their fleets of McDonnell Douglas MD-11 planes "out of an abundance of caution" following a deadly crash at the UPS global aviation hub in Kentucky, the companies announced late Friday.

The MD-11 aircrafts make up about 9% of of the UPS airline fleet and 4% of the FedEx fleet, according to the companies.

"We made this decision proactively at the recommendation of the aircraft manufacturer," a UPS statement said. "Nothing is more important to us than the safety of our employees and the communities we serve."

FedEx said in an email that it will be grounding the aircrafts while it conducts "a thorough safety review based on the recommendation of the manufacturer."

Boeing, which merged with McDonnell Douglas in 1997, did not immediately respond to an email from The Associated Press asking the reasoning behind the recommendation.

The crash Tuesday at UPS Worldport in Louisville, Kentucky, killed 14 people, including the three pilots on the MD-11 that was headed for Honolulu.

The cargo plane was nearly airborne when a bell sounded in the cockpit, National Transportation Safety Board member Todd Inman said earlier Friday. For the next 25 seconds, the bell rang and the pilots tried to control the aircraft as it barely lifted off the runway, its left wing ablaze and missing an engine, and then plowed into the ground in a spectacular fireball.

The cockpit voice recorder captured the bell, which sounded about 37 seconds after the crew called for takeoff thrust, Inman said. There are different types of alarms with varying meanings, he said, and investigators haven't determined why the bell rang, though they know the left wing was burning and the engine on that side had detached.

Inman said it would be months before a transcript of the cockpit recording is made public as part of that investigation process.

Jeff Guzzetti, a former federal crash investigator, said the bell likely was signaling the engine fire.

"It occurred at a point in the takeoff where they were likely past their decision speed to abort the takeoff," Guzzetti told The Associated Press after Inman's news conference. "They were likely past their critical decision speed to remain on the runway and stop safely. ... They'll need to thoroughly investigate the options the crew may or may not have had."

Dramatic video captured the aircraft crashing into businesses and erupting in a fireball. Footage from phones, cars and security cameras has given investigators evidence of what happened from many different angles.

Flight records suggest the McDonnell Douglas MD-11, built in 1991, underwent maintenance while it was on the ground in San Antonio for more than a month until mid-October. It is not clear what work was done.

The UPS package handling facility in Louisville is the company's largest. The hub employs more than 20,000 people in the region, handles 300 flights daily and sorts more than 400,000 packages an hour.

UPS Worldport operations resumed Wednesday night with its Next Day Air, or night sort, operation, spokesperson Jim Mayer said.