Lawrence B. Mulloy, the NASA engineer often singled out for criticism for his role in the Challenger accident, acknowledged for the first time yesterday that "we took a step too far" and said he just "wasn't smart enough" to solve the problems that led to the disaster.

He said in a telephone interview that if he and his colleagues had properly analyzed the information available to them before the launch, they would have realized that cold weather could cause the joint to fail. A sound analysis "just wasn't done and it should have been," he said.

In a related development, President Reagan said at his news conference last night that no specific individuals should be held accountable for the accident. "I don't believe there was any deliberate or criminal intent in any way on the part of anyone," Reagan said, adding that the accident was the result of "complacency . . . a carelessness that grew out of success."

The decision of the commission investigating the accident not to hold individual NASA managers responsible has come in for sharp criticism from some members of Congress since the report's release Monday.

Mulloy, who was in charge of the shuttle's rocket boosters at the time they caused the Challenger accident, continued to defend his own role in the launch, and strenuously denied suggestions by the Rogers Commission that he had been untruthful in commission hearings.

He said he agreed with essentially all of the Rogers Commission's findings about the accident, including the conclusion, which he called "well founded," that National Aeronautics and Space Administration managers had warnings of the problems nearly a decade ago.

"Given what I know in the four months since we had that failure, looking back in hindsight, there is a point . . . where I think we took a step too far," he said. He identified that point as the spring of 1985, after a flight in which one of the shuttle's boosters suffered particularly severe erosion.

But he said that singling him out for special responsibility in the accident, as Sen. Ernest F. Hollings (D-S.C.) did Tuesday in congressional hearings, is unfair because his own judgments are "taken to three other levels for review . . . . "

"I will say I discharged my job as a project manager responsibly within the bounds of authority I had," Mulloy said in a 45-minute telephone interview from his home in Huntsville, Ala. "Given the system, which Chairman William P. Rogers said is at fault, I don't really understand how I can be held singularly responsible as a sole individual."

"I regret the accident ever occurred," said Mulloy, who, the night before the launch, argued with the contention of Morton Thiokol engineers that cold weather might lead the faulty joints to fail. "Given the information I had at the time . . . I can't say that anything different would have been done. I do regret that it wasn't."

"I should have somehow highlighted more in some way that this the joint was a critical problem . . . Obviously it's a hindsight judgment . . . I do not take exception to the commission's judgment."

The report also implied that Mulloy lied to the commission, calling his testimony "disturbing . . . . C ontrary to the testimoy of Mulloy , the seriousness of concern was not conveyed."

"I am most distressed," Mulloy said last night, "by the media interpretation that the commission has accused me of perjury. That is the most distressing thing, because that is harmful, and it is not true." Since the accident, Mulloy has been transferred to a new position at the Marshall Space Flight Center as assistant to the director of science and engineering for the center.

Asked why the commission ended up understanding the joint better than the engineers who designed it and were responsible for it, Mulloy said: "I wasn't smart enough, the people who advised me weren't smart enough, the contractor wasn't smart enough . . . the people who review my activities weren't smart enough . . . . No one was smart enough to realize what was necessary."

It is not that the commission members are so much smarter, he said, but "additional data has become available. The most significant data is that a joint had failed. Knowing that something has failed, one might be able to recognize better what might have precluded it failing ."

"The most important thing to realize about my role," he said, "is that I functioned responsibly given the responsibilities of my position. I was reliant on the information and recommendations that came to me, applying my own judgment, and then having that judgment tested by three levels of management above me."

The Rogers Commission report, he said, "is a very good piece of work. I haven't found anything yet that I have read that I take issue with."

Pressed at his news conference about whether he agreed that specific shuttle managers need not be held responsible for the accident, Reagan said, "Yes, I do."

"I think that with the great record of success that NASA has had going . . . I think there was a complacency there . . . . " he said. "It's something that has to be corrected before one of those shuttles takes off again. But I think it was just a carelessness that grew out of success.

"Maybe part of it the accident was also due . . . " Reagan concluded with a chuckle, "to the balmy climate of Florida, and that it was difficult for anybody to believe that they had had a cold snap that could render that O-ring dangerous."

Top NASA officials, embracing the Challenger commission report as a "road map" for recovery, said yesterday that problems in redesign and testing of new shuttle hardware could postpone the resumption of shuttle flights beyond the agency's July, 1987 target and into 1988.

During the second day of hearings by the House Science and Technology Committee on the Jan. 28 Challenger accident, shuttle director Rear Adm. Richard Truly also said that all shuttle flights should have been stopped as early as November 1981 when erosion of the rocket booster seals was first observed after a flight he piloted.

"It should have been worked as a major technical problem at the the top of table," said Truly, who piloted the orbiter Columbia on the second shuttle flight. "If I had known then what I know now, I would have stopped the program and fixed the problem."