The NTSB is investigating how a piece of the engine broke off and smashed the window, leaving one passenger seriously injured.

Its preliminary report into the 10 July incident found that flight crew had reported four suspected bird strikes to the aircraft’s No 2 engine in the preceding 12 months.

"Bird remains were reportedly found in two of the cases," the agency said, although no damage was located during subsequent maintenance.

A loud bang was picked up by the cockpit voice recorder about nine minutes and 20 seconds after the start of the take-off roll, and examination of the engine found fractured fan blades, with three loose fan blade fragments recovered.

The CFM engine, built by a venture between France’s Safran and US-based General Electric, had been inspected in May with no findings, according to the US probe.

Ryanair declined to comment, CFM said it was assisting the investigation, and Boeing deferred to the NTSB. The agency is also examining whether the incident resembles earlier failures, including an April 2018 Southwest Airlines engine failure that killed a passenger partially sucked out of a window.