The gene therapy researcher who oversaw the medical experiment that killed a Tucson teenager in September acknowledged yesterday that he treated the young man even though Jesse Gelsinger's blood tests were "slightly" abnormal.

However, he said, he "felt comfortable with the decision" to proceed with the infusion of gene-altered viruses because he believed that Gelsinger's condition had been stabilized with medicines given the night before.

Food and Drug Administration officials have said that decision was one of several serious lapses uncovered in an ongoing federal investigation of the University of Pennsylvania study, which was testing the safety of a new genetic treatment for a rare inherited liver condition.

Agency officials reiterated that position during a news conference yesterday following a special gene therapy meeting at the National Institutes of Health in Bethesda.

The researcher, James M. Wilson, who was attending the meeting, then read a brief, prepared statement that provided his justification for treating Gelsinger.

Wilson said that Gelsinger's blood ammonia levels--a measure of liver vitality--had been within an acceptable range when the teenager was first enrolled in the study. He said it was his understanding that this was the time the FDA rules applied to. In the days preceding the treatment, those levels rose to a level higher than the maximum allowable under rules agreed to in advance by the researchers and the FDA. Gelsinger was given conventional medicines the night before he was infused with the viruses.

It remained unclear yesterday whether the medicines he received the night before the experiment successfully lowered his ammonia levels, and Wilson refused to answer questions.

The FDA yesterday also said that four volunteers--more than previously reported--had experienced serious "Grade 3" toxic reactions before Gelsinger was enrolled, but the researchers alerted the agency only to the first two.

Wilson said he had provided all patient data to the agency, but the FDA countered that the researchers had neither specifically flagged the worrisome results nor halted the study as required.

Also yesterday, NIH officials said that the Penn researchers had not reported those serious side effects to the NIH Office of Biotechnology Activities, as required. That oversight office's first notice of any problems in the NIH-funded study was a report from the researchers of Gelsinger's death.

In an impromptu presentation after the meeting, Gelsinger's father, Paul, defended Wilson. "I touched souls with this man," Gelsinger said. "There is nothing wrong with Wilson."

He blamed the FDA for failing to stop the experiment before his son's death.