Jailed Islamic extremists are taking advantage of few available Muslim chaplains and scarce religious monitoring programs in U.S. prisons to breed terrorists with violent interpretations of the Koran, a study released Tuesday shows.

State and local prison officials struggle to track radicalized behavior by inmates or religious counselors, the joint study by George Washington University and the University of Virginia found. And many prisons cannot afford preventive programs, including in California, where officials reported "that every investigation into radical groups in their prisons uncovers new leads, but they simply do not have enough investigators to follow every case of radicalization."

"Radicalized prisoners are a potential pool of recruits by terrorist groups," concluded the study, released at a Senate Homeland Security and Governmental Affairs Committee hearing on homegrown terrorists. "The U.S., with its large prison population, is at risk of facing the sort of homegrown terrorism currently plaguing other countries."

An estimated 2 million people are imprisoned in the United States; 6 percent of them are Muslim, according to the Federal Bureau of Prisons.

Daveed Gartenstein-Ross, a counterterrorism consultant, told senators that "chilling" interpretations of the Koran in English were given to prison inmates when he worked for the al-Haramain Islamic Foundation, an international charity that served as a major Al Qaeda financier.

The readings urged Muslims "to wage war against non-Muslims who have not submitted to Islamic rule," Gartenstein-Ross said in prepared testimony to the Senate panel.

"I know of only a few instances in which prisons rejected the literature we attempted to distribute — and it was never because of the literature's radicalism," said Gartenstein-Ross, who has since left the charity and converted to Christianity.

Prisons have long been considered recruiting stations for gangs and, more recently, terrorists, but little has been done throughout government to combat them. The Senate hearing came as law enforcement and intelligence officials focus on finding out how and why extremist homegrown sympathizers cross a line to become operational terrorists.

The panel's chair, Sen. Susan Collins, called the matter "an emerging threat to our national security." Added Sen. Tom Carper, "While homegrown Islamic terrorism might not be as much of a threat as in say, Europe or some other places, we ignore the threat that does exist at our peril."

The report cited several high-profile cases of terrorists who became radicalized while incarcerated, including British shoe bomber Richard Reid. It also noted what authorities call a foiled plot of a potential shooting rampage against California military facilities, synagogues and the Israeli Consulate in Los Angeles by followers of Kevin James, who founded the radical group Jamiyyat Ul-Islam Is-Saheeh, or JIS, as an inmate at California State Prison in Sacramento.

Researchers interviewed federal, state and local prison officials, religious counselors and counterterror authorities in four states — California, New York, South Carolina and Ohio — and the District of Columbia. They concluded that federal prison authorities have made significant strides in collecting and sharing information to help monitor whether inmates are becoming radicalized.

But state and local prison officials have largely relied on contractors and volunteers to lead Islamic services because of a lack of well-trained Muslim chaplains, the report found. In New York, that led to several cases of "imams espousing violent views," it said.

The report noted a 2004 study that found that about half of 193 prisons surveyed supervised religious services or monitored them with video or audio recorders. "In the absence of monitoring by authoritative Islamic chaplains, materials that advocate violence have infiltrated the prison system undetected," it found.