Investigators say when they looked at the plane's engine, some parts were loose and appeared to be installed incorrectly, with one "allowing fuel to leak from the fitting."

An American Airlines flight experienced an engine fire due to loose parts installed in the wrong direction and fuel leaks, according to an NTSB investigation.

The Boeing 737 that caught fire at Denver International Airport in March had parts incorrectly installed which lead to fuel leaks, according to a preliminary report released Thursday by the NTSB.