Investigators release preliminary findings on the United Airlines flight engine failure. Here's what we know

(CNN)The investigation into the engine failure of a Boeing 777 could take more than a year, officials said, but already the picture is becoming clearer about what happened to the United Airlines flight on Saturday.

According to investigators, the Pratt & Whitney engine failed minutes into United Airlines flight 328, headed from Colorado to Hawaii, leaving a mile of debris in its wake. After the incident, United grounded all of its Boeing 777s powered by PW4000 series engines.

A preliminary investigation indicates the damage was "consistent with metal fatigue," according to information from the National Transportation Safety Board (NTSB), which has prompted federal regulators to reexamine engine inspections.

Engine on United Airlines flight was showing signs of metal fatigue, NTSB says

"Pratt & Whitney is actively coordinating with operators and regulators to support the revised inspection interval of the Pratt & Whitney PW4000 engines that power Boeing 777 aircraft," the company said. "Any further investigative updates regarding this event will be at the discretion of the NTSB. Pratt & Whitney will continue to work to ensure the safe operation of the fleet."

Both the Federal Aviation Administration (FAA) and the NTSB are investigating the incident. Here is what we know so far.

This image taken Monday shows the damage to fan blades in the number 2 engine of United Airlines flight 328.

What happened on the flight

"United 328 Heavy -- Mayday Mayday ... Denver departure. United 328 Heavy Mayday. Aircraft just experienced engine failure -- need to turn immediately," the air traffic audio from the plane rang out on Saturday.

The PW4000 engine has 22 blades, investigators said, one of which was found lodged inside the jet engine's containment ring. Another was found in a soccer field in Broomfield, Colorado.

One of the blades in the plane's right engine broke free at the hub, likely hitting another that was broken mid-span, investigators said Monday. The former shows damage "consistent with metal fatigue," NTSB Chairman Robert Sumwalt said.

Pilot: What happens in an emergency like United 328

The flight fuselage also had damage to a non-critical composite piece designed to make the plane more aerodynamic, Sumwalt said.

On the plane, passengers saw the engine completely stripped of its outer casing, according to video from passenger Travis Loock.

The mood was tense, Loock told CNN, but everyone was "very calm" on board as the pilot came on to say they would be landing in four minutes.

"My daughter was sitting on the window and ... I was just like, 'don't look, like let's let's close it up and let's just pray,'" passenger Brenda Dohn said.

The trail of damage

None of the passengers were injured. Nor were residents around Broomfield from the falling debris that stretched a mile.

"We dispatched police officers and within minutes we actually were on scene of some of these homes, and we actually saw some of these large pieces of debris," Broomfield Police spokesperson Rachel Welte told reporters Saturday.

United Airlines flight suffers engine failure, sending debris falling on neighborhoods outside Denver

Looking at the debris field and how busy the area was, Welte said, "the fact that we are still not getting reports of any injuries is absolutely shocking at this point."

"This park on a day like today, when it's not as cold as it was last weekend, we could have hundreds of people here."

The cab of Broomfield resident Kirby Klements' truck was a stark reminder that damage was done: an engine cowling landed and caved in the cab, according to CNN affiliate KCNC.

"I'm sitting here looking at this piece of junk sitting in my driveway thinking, 'Oh my God, what the hell am I going to do now?'" Klements said of the vehicle he put so much time and money into.

Kieran Cain was playing basketball with his kids Saturday afternoon at an elementary school in greater Denver when he heard what sounded like a sonic boom and looked up.

"We could see there was a giant black cloud of smoke high up in the sky, immediately followed by, you know what looked like pieces of the aircraft," Cain told CNN. "Basically, a shower of things that were falling out of the sky."

It may be a while before members of the community get a payout from United Airlines, but most standard insurance plans should cover "objects falling from the sky," Carole Walker with the Rocky Mountain Insurance Information Association told KCNC.

United Airlines flight 328 experienced a right engine failure after takeoff from Denver International Airport.

Where the investigation goes next

The engine in question will be removed from the aircraft for additional inspection, Sumwalt said.

A section of a blade will be examined Tuesday after being flown by private jet Monday night to a Pratt & Whitney lab, where investigators believe they can determine for how long the blade was damaged before it failed.

Airlines ground dozens of Boeing 777 planes after engine failure over Denver

The NTSB arrived in Colorado on Saturday and began removing pieces of debris to a hangar for further examination, a tweet from the Broomfield Police Department said. Investigators for the agency in Washington, DC, have begun an analysis of the flight's cockpit voice and flight data recorders and have not yet inspected United Airlines maintenance records.

In the wake of the engine failure, Boeing has recommended suspending the use of 777s that have a Pratt & Whitney 4000 engine, and United Airlines has already pulled them following the incident. The FAA also issued an emergency order earlier Sunday saying it would be stepping up inspections of Boeing 777 airplanes equipped with certain Pratt & Whitney PW4000 engines.

CNN's Gregory Wallace, Pete Muntean, Hollie Silverman, Alta Spells, David Williams, Konstantin Toropin, Chuck Johnston, Andy Rose, Gregory Clary and Lucy Kafanov contributed to this report.