The US Air Force has indefinitely grounded all Boeing Co. F-15 fighter jets after a plane flown by the Missouri Air National Guard crashed last week during a training exercise. "The cause of that accident is still under investigation, however, preliminary findings indicate that a possible structural failure of the aircraft may have occurred," the Air Force said in a statement released late Sunday. Maj. Cristin Marposon, an Air Force spokeswoman, said Monday that the country's fleet of 676 F-15s, including mission critical jets, were grounded Nov. 3 for "airworthiness concerns." While the F-15 is grounded, the Air Force said it would rely on Lockheed Martin Corp.'s F-16 fighter jet, and other aircraft, for routine missions. However, the F-15 will remain available for combat or other emergency situations, Marposon said.