New York, Apr 19 (IANS): The US aviation authorities have ordered inspections of fan blades in jet engines after a mid-air explosion punctured a Southwest Airlines' window, killing a passenger on Tuesday.

Fan blades that have undergone a certain number of flights will be given ultrasonic tests, the BBC quoted the federal authorities as saying.

An initial investigation found evidence of metal fatigue where a fan blade had broken off, according to the US National Transportation Safety Board (NTSB).

Chairman Robert Sumwalt told reporters that the fan blade had a second fracture about halfway along its length. He could not say if the incident indicated a fleet-wide issue with the Boeing 737-700.

The Federal Aviation Administration (FAA) did not say how many engines would be inspected. It said that any fan blades that failed the inspection would have to be replaced, the BBC reported.

Sumwalt also said a casing on the engine is meant to contain any parts that come loose but, due to the speed, the metal was able to penetrate the shell.

The CFM56-7B engine is in use on more than 8,000 Boeing 737 planes, the manufacturer said.

They were developed by French-US joint venture CFM International, which says it is the "world's leading supplier of jet engines for single-aisle aircraft".

CFM's parent companies are sending 40 technicians to help Southwest inspect engines. French officials are travelling to the US to assist with the investigation.

Southwest Airlines Flight 1380, en route from New York to Dallas with 149 people on board, was forced to make an emergency landing at Philadelphia airport on Tuesday.

A female passenger died after being sucked into the hole and nearly blown out of the cabin.