ASHINGTON, May 4 — Groups promoting extremist brands of Islam have gained a foothold in American prisons, and counterterrorism officials believe Al Qaeda are likely to try to use the prisons "to radicalize and recruit inmates," according to a Justice Department investigation.

In a report from the Justice Department inspector general's office, investigators said safeguards were so loose in the 105 federal prisons that inmate chapels "remain vulnerable to infiltration by religious extremists." A copy of the report, to be released on Wednesday, was obtained by The New York Times.

The investigation grew out of concerns among members of Congress that groups training Muslim chaplains had terrorist ties and were breeding extremism. But the investigation found that the problem of "radicalized" prayer sessions was less a reflection of the chaplains than of unsupervised inmates who were allowed to lead their own worship meetings.

"Too many opportunities for abuse of this practice exist," the report found.

The inspector general's report, the first detailed look into how the federal prisons have dealt with extremist beliefs since the Sept. 11 attacks, will likely prove controversial among Muslim leaders, who say they have been subjected to unfair scrutiny and criticism because of their religious beliefs. Several groups that have trained Muslim chaplains have vigorously denied charges of terrorist links, and Muslim leaders point out that charges linking a military chaplain at Guantánamo Bay to possible terrorism largely collapsed.

The inspector general concluded that while the problem of terrorist recruitment in the federal prisons was not necessarily widespread, officials needed a number of systemwide improvements to ensure tighter control. Prison officials said Tuesday that they had already moved to fix some problems identified in the report by demanding more information about outside groups that train chaplains and by improving communications with the Federal Bureau of Investigation. The report found that prison officials received sparse information from the F.B.I. about inmates or chaplains who may have terrorist connections.

"We understand the seriousness and the risks inherent with extremist chaplains, contractors or volunteers," said Dan Dunne, a Bureau of Prisons spokesman. "And we've made significant changes since the review was initiated to better screen religious service providers."

A classified addendum to the report details cases in which counterterrorism officials assert that people leading prison prayer sessions — including authorized chaplains, volunteers and inmates — may have ties to terrorist groups.

In a briefing Tuesday for Congressional officials, the inspector general's office said it found evidence that volunteers leading prayer services had been linked to people who showed up on terrorist watch lists, and that people associated with Al Qaeda had already managed to recruit support within the federal prisons, said Senator Charles E. Schumer, Democrat of New York.

Federal prison officials "were putting out the welcome mat to any group that wanted to infiltrate the prisons," Mr. Schumer said. "There was virtually no vetting of who would become a chaplain or a volunteer, and there was virtually no supervision. It was an invitation to danger."

Senators Schumer and Jon Kyl, Republican of Arizona, requested the investigation and held a hearing last year after concerns focused on the case of a Muslim chaplain, Warith Deen Umar, who had run New York State's Islamic prison program and was a consultant in the federal prisons. Mr. Umar was banned from the state prison program after he reportedly expressed admiration for the Sept. 11 hijackers and espoused a radical brand of Islam, but he maintained he was misquoted.

Senator Kyl said the inspector general's findings confirmed his concerns about the spread of extremist messages in the prison system, where Muslims represent an estimated 9,000 of the 150,000 inmates.

"There's a concern that groups may already be radicalizing people in prison," he said. "Some of the findings are troubling, and clearly there is work to be done."

The report found that chapels are among the few areas in federal prisons where large numbers of inmates can meet and talk, and it noted that several high-profile terrorist suspects had been drawn to Islam while in prison. Chaplains sometimes supervise the prayer sessions with no guards present, and some prayer sessions are conducted partly in Arabic, the report said.

Although some chapel services are videotaped, prison officials admitted that they might not be in a position to detect radical religious messages. "Not a whole lot of folks are in tune with that stuff," said an associate warden quoted in the report.