Miami ? After two years of work and more than $1.5 billion in modifications, the space shuttle and its crew still could be endangered by the same debris problem that destroyed Columbia, NASA conceded Friday as it again delayed the spaceship’s return to flight.

Officials announced that shuttle Discovery, on the launch pad and scheduled for liftoff May 22, will be rolled back 4.2 miles to the hangar for additional repairs to its huge external fuel tank. The mission to test new safety measures and deliver supplies to the International Space Station will be postponed until at least July 13.

NASA’s top administrator called the delay “another routine launch slip,” but others wondered why the latest problem wasn’t identified and corrected before Discovery was on the pad. Many outside observers and some NASA engineers fear the agency is rushing the shuttle back to space.

The main issue again is debris — in the form of foam insulation or shards of ice — that could break off the fuel tank during launch and inflict catastrophic damage on the orbiter, the airplanelike component that will carry Discovery’s seven astronauts.

The February 2003 loss of Columbia and its seven crew members was caused by a suitcase-sized, 1.67-pound piece of foam that came loose and punched a hole in that shuttle’s left wing.

A NASA spokesman said the chunk of foam came from a spot just a few feet from the latest point of concern on the fuel tank, where ice can form and turn into shrapnel after the tank is filled with supercold fuel.