Washington ? Federal law enforcement officials are concerned that al-Qaida is targeting what authorities fear is an increasingly fertile breeding ground for Islamic extremism: U.S. prisons.

In the past year, officials have suspected that al-Qaida and other extremists have been reaching out to U.S. prisoners through reading material and personal contacts to try to form a base from which to gather information, funding and recruits.

Although al-Qaida’s efforts seem only loosely organized, one former top FBI official said prisoners were one of the three groups that most troubled the bureau as it monitored terrorists’ efforts to recruit followers and inflame anti-American fervor.

The two other groups are those who have trained at al-Qaida camps in Afghanistan and those who have joined radical mosques here and abroad.

Prisons are “a captive audience of people already willing to use violence,” the official said. “It’s fertile ground.”

There is no evidence that al-Qaida has yet succeeded in recruiting many prisoners in the United States or overseas. But authorities point to the case of Jose Padilla, a former Chicago gang member who is accused of plotting to set off a radioactive “dirty” bomb in the United States. Padilla had converted to Islam while in a Florida jail.

In addition, Richard C. Reid, who pleaded guilty to trying to blow up a jetliner with a bomb in his shoe, converted to an extreme form of Islam while in prison in London.

Officials say that while the cases of Padilla and Reid are rare, they can’t afford to ignore the extremism that seems to be brewing in the prison system.

Charles E. Mandigo, special agent in charge of the FBI’s Seattle field office and an authority on terrorists, said al-Qaida was not likely trying to assemble the next band of hijackers or suicide bombers out of groups of incarcerated U.S. convicts.

What’s more probable, he said, is that terrorists are looking to establish a support structure in the United States. Such connections could help al-Qaida gather details about targets and build a network of contacts.

Terrorists, Mandigo said, regard U.S. prisons as an ideal place to find groups hostile to the federal government and for radical propaganda to flourish.

Mahdi Bray, who runs the National Islamic Prison Foundation, a Washington-based outreach group, said the foundation received thousands of books and pamphlets from Arabic nations to distribute to prisoners. Each year, it sends more than 20,000 Qurans and other books to those incarcerated.

But, he says, the group has also received extremist literature, sometimes from Pakistan and Saudi Arabia, filled with anti-American vitriol. “When we looked at some of them, we said, ‘Hell no,'” Bray said. “I just won’t distribute those.”

For the FBI, the need to combat this kind of extremist influence strains resources. The work involves hunting not would-be terrorists, but rather their sympathizers, who might not be intending to carry out terrorist acts.

“It eats up a lot of our time,” Mandigo said, adding, though, “Even if they never do anything, we can’t afford to ignore them.

“At a minimum, (prison recruiting) provides a smoke screen” that could divert authorities’ resources and attention away from a terrorist plot.

Yet there is risk, too, in ignoring any al-Qaida support network in the United States. It takes just one disgruntled sympathizer to carry out a terrorist act. Newly recruited radicals, officials say, could become frustrated with playing a minor role or with al-Qaida’s pattern of waiting years before attacking.

Muslim clerics who work with U.S. prisoners say Islam has enjoyed a resurgence in recent years, giving many convicts a moral purpose and teaching them to pursue discipline and self-sacrifice.

At least 10 percent of prisoners, and sometimes up to 20 percent, say they are followers of Islam, according to state and federal prison statistics.

But some Muslims acknowledge worries about potentially ominous signs of Islamic extremism: senior clerics, or imams, who have traveled to Arabic countries for “training” and radical literature that has flowed into prison cells.

“I call it the unholy alliance,” said Faheem Shuaibe, imam at a mosque in Oakland, Calif., who has spent much time working in California prisons.

Shuaibe says he believes al-Qaida and other extremists have been targeting black prisoners, who they hope will turn against the country. Most Muslims in U.S. prisons are black.

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“In some ways, the psychological impact of racism is similar to the feelings of oppression (Muslim radicals) have toward the United States,” Shuaibe said. “It’s the human reaction to an overwhelming power. And where else can you find a pool of people willing to use violence to reach their goals than prison?”

At the same time, Shuaibe suggested, the work of imams who teach Islamic messages of peace and self-growth helps neutralize the hate-filled messages some prisoners are receiving.

The prison environment in recent years also has created obstacles for investigators as the number of groups with extremist or separatist ideas has risen.

“The problem (of al-Qaida in prisons) is further exacerbated by the fact that we used to keep prisoners busy,” said Bob Fosen, a professor at American University and a former top official in New York and California’s prison systems. “Even the untrained person would notice the idleness that’s all over the place. It’s exactly the kind of environment you don’t want.”

The United States, Fosen said, largely abandoned the idea of prison rehabilitation in the 1970s ” and with it, much of the schooling, work and self-help programs that kept prisoners occupied. “It promotes discontent,” he said of today’s prison culture. “We’re not a species that’s good at doing nothing.”

Most black prisoners, which Bray’s foundation is closest to, are not the ready recruits al-Qaida thinks they are, Bray said.

“They thought, ‘African-Americans are the most mistreated ” recruit them.’ But it didn’t work. Ultimately, people see through it.

“You can’t understand American culture from a cave in Afghanistan,” he said, “and you certainly can’t understand African-American culture.”