After the deadly shooting rampage in Tucson, Sen. Patrick J. Leahy (D-Vt.) was asked to reflect on his own experience as the would-be target of an assassin. That’s when he let slip something that he rarely talks about publicly: He has never accepted the FBI’s decision to close the case in the series of anthrax-laced letters mailed to public officials in fall 2001.

“I still wonder who sent it and why they sent it,” the Judiciary Committee chairman told a crowd gathered last month at the Newseum in Northwest Washington to hear his 2011 legislative agenda.

More than a month later, Leahy was given fresh evidence this week that the science in the case was not airtight, reopening emotional wounds 91 / 2 years after letters sent to him and then-Senate Majority Leader Thomas A. Daschle (D-S.D.) helped cause the deaths of five people and sickened 17 others. On Tuesday, the National Research Council questioned the efficacy of the genetic testing used by the FBI to allege that a Fort Detrick scientist had acted alone in mailing the deadly letters to Capitol Hill and media outlets.

The FBI stood behind its investigation, concluding that Bruce E. Ivins, who worked at the Army base that conducted high-level chemical and biological tests, acted on his own. Ivins committed suicide in July 2008 as he was about to be indicted, but his lawyer has maintained his innocence. Officials said Tuesday that the totality of evidence, including Ivins’s unexplained absences from work around the time of the anthrax mailings, convinced investigators that Ivins was still the “perpetrator.”

Leahy has generally declined to comment on the anthrax investigation, telling The Washington Post that he has tried not to discuss it much “because it affects me so. . . . ” his voice trailing off without finishing the thought.

But in a brief interview Tuesday, he said he has “extreme doubts” about the case. “I’ve expressed those concerns to the FBI ,and this report adds to those concerns,” Leahy said.

The handling of the almost nine-year investigation, dubbed “Amerithrax” by the FBI, has never sat well with Congress. On Tuesday, Sen. Charles E. Grassley (R-Iowa), a longtime FBI critic, called for an independent review, and Rep. Rush D. Holt (D-N.J.), from whose district the letters to Leahy and others were mailed, called for Congress to create an independent commission to examine the entire case.

For years the targets of the Capitol Hill-bound letters felt shut out of the anthrax investigation, which included an official blackout of any congressional briefings ordered by the Bush administration in 2004. Now, however, there are key figures throughout the Obama administration, and Leahy and Grassley are the top senators on the Judiciary panel with FBI oversight.

A huge collection of former Daschle staff members now occupy the highest echelon of posts in the Obama administration. Pete Rouse, then the chief of staff to the majority leader, is President Obama’s deputy chief of staff. Laura Petrou, the top aide in Daschle’s suites in the Hart Senate Office Building, where an anthrax letter was opened Oct. 15, 2001, is now chief of staff at the Department of Health and Human Services. Mark Patterson, then Daschle’s top policy adviser, is chief of staff at the Treasury Department.

Daschle declined to comment Tuesday, but he has criticized the handling of the case, which included several years of focusing on a scientist who was later cleared.

Leahy’s only other public comments came in mid-September 2008, when FBI director Robert S. Mueller III testified before the Judiciary Committee just six weeks after Ivins died. In a brief aside, Leahy said there were “others who could be charged with murder.” Mueller merely said, “I understand that concern.”

In an interview after his Newseum comments last month, the 36-year vet­eran of the Senate expanded on his concerns about the case. Both as chairman of the committee overseeing the FBI and as a target of one of the letters, Leahy has been granted unique access to the investigation. He has seen any file he wanted and has studied the case.

“The director has, to his credit, answered every single question I’ve ever asked him. He’s called me at home; he’s met with me privately,” Leahy said.

To Leahy and others on Capitol Hill, a key issue is whether other scientists helped Ivins or handled the anthrax itself. The council’s report, commissioned by the FBI, “did not definitively” demonstrate a link between the anthrax used in the deadly letters and that in a flask in Ivins’s lab at the Army base in Maryland. It also questioned whether Ivins, as the FBI has alleged, was the only scientist at the base with the expertise to handle those anthrax spores.

That left a sense of bewilderment among current and former officials on Capitol Hill.

“It is mystifying. Given the limited number of people who have experience with anthrax, you just wouldn’t think it would be this hard,” said another official who had been briefed on the Amerithrax investigation. The official requested anonymity to discuss a sensitive probe.

Last month, Leahy said there was no question that others were involved in the anthrax attack, at least in helping Ivins hide his work from authorities.

“Were there people who at the very least were accessories after the fact? I think there were,” he said. Leahy also finds it strange that one person would target such an odd collection of media and political figures in the anthrax letters, a motive that has never been fully explained by the FBI or Justice Department.

“Why would he send one to Tom Brokaw, to Tom Daschle, to me, to the man at the National Enquirer in Florida?” Leahy asked.

“They have to make their decisions; I have to make mine. In my mind, it’s not closed,” he said. “Call it an old prosecutor’s instinct.”