Federal investigators gave the strongest sign yet that manufacturing problems, including a pattern of misaligned rivet holes and loose bolts, likely played a major part in the recent midair fuselage rupture of a Southwest Airlines Co. jetliner.

Updating its probe into why a five-foot section of the aging Boeing 737 ripped open earlier this month, the National Transportation Safety Board on Monday laid out a series of preliminary findings focused on production imperfections that may have resulted in dangerous structural cracks in the jet.

Some of the defects were discovered in a section of the plane's aluminum skin several feet from the rupture, including gaps between rivets and their holes, which didn't have the proper shape, according to the board's statement.

The April 1 event, which shocked the airline industry and spooked many travelers, occurred as the 15-year-old plane was cruising at 34,000 feet from Phoenix to Sacramento, Calif., with 122 people aboard. The twin-engine jet experienced a sudden cabin depressurization, but no one was seriously injured and the pilots made an emergency landing at a military air base in Yuma, Ariz.

Without identifying a specific cause, the safety board disclosed that it found "fatigue cracks emanating from at least 42 of the 58 rivet holes" along the tear. Aircraft fuselages experience stress from pressure changes during each takeoff and landing, and those can cause microscopic cracking and, ultimately, possibly result in dangerous metal fatigue.