Washington ? Lessons learned from the 1986 explosion of the space shuttle Challenger were “undone over time” by NASA managers long before the disintegration of its sister spacecraft Columbia in a fiery return to Earth in February.

Eighty-seven shuttle missions after Challenger, investigators are wondering why.

“It didn’t get fixed last time,” said Steven B. Wallace, a member of the Columbia Accident Investigation Board. “There has to be a different approach now.”

The board, in its final report released Tuesday, found striking parallels between the Columbia and Challenger accidents. It said NASA must overcome its rigid culture of stifling dissent, discounting safety problems and worrying too much about flight schedules if it hopes to safely return the shuttle to flight.

“Despite all the post-Challenger changes at NASA and the agency’s notable achievements since, the causes of the institutional failure responsible for Challenger have not been fixed,” the board wrote, adding that unless flaws are fixed, “the scene is set for another accident.”

NASA officials have vigorously objected to comparisons between Challenger and Columbia, arguing that since the Challenger accident even lower-level employees have authority during preflight reviews to raise safety concerns that can halt a launch.

One board member, Sheila Widnall, said Tuesday that she “wanted to make sure we were not the second report to be joined on a shelf, to be joined by a third report.” Another board member, Maj. Gen. John Barry, found “echoes of Challenger.”

With both shuttles, the board concluded, NASA managers became dangerously inured to lingering safety problems — erosion of a crucial O-ring component in 1986 and shedding foam in 2003 — and permitted flights that were fundamentally unsafe. Researchers describe the tendency to become increasingly comfortable with risks as the “normalization of deviance.”

In one of the few dramatic new disclosures in the board’s report, a top shuttle manager, Linda Ham, confided in an e-mail before Columbia’s breakup that NASA’s rationale for allowing continued flights before resolving a problem with breakaway insulating foam was “lousy then and still is.”

Yet she never sought to intervene and spiked a request for telescopes or spy satellites to capture images of possible damage to Columbia’s wing.

The board compared that decision to NASA managers’ decision to permit Challenger’s launch in 1986 despite pointed concerns about heat erosion in O-rings. “Neither problem was defined as a show-stopper,” the board wrote.