DALLAS — Inspections of some older Boeing jets have turned up structural cracks in more than three dozen of them, raising a new safety issue for the company already dealing with two deadly crashes involving a newer version of the same plane.

Boeing said Thursday that airlines worldwide have inspected 810 planes following an order from U.S. safety regulators. Of those, 38 – or 5% – had “findings” requiring repairs.

Airlines are under orders to inspect certain Boeing 737 NG planes for cracking in a part that helps keep wings attached to the fuselage.

Boeing declined to identify which airlines found problems, but Brazilian carrier Gol said it has grounded 11 planes so far, and U.S.-based Southwest Airlines grounded two.

The NG is a version of the popular 737 that has been produced since the 1990s. Boeing is replacing it with the 737 Max, but those planes have been grounded worldwide since March.

The Federal Aviation Administration gave airlines seven days, ending Thursday, to inspect 737 NGs that had made at least 30,000 flights. A much larger group of planes with slightly fewer flights must be inspected over the coming months.

Boeing is analyzing airline inspection results, and its technical experts are deciding the best repair plan.

The constant pressure and forces during flight can cause metal fatigue on planes, and it is not unusual for regulators to require inspections of a specific component or area. In one case, the FAA in 2011 ordered periodic inspections of the top of some Boeing 737s after a hole tore open during a Southwest flight.