WASHINGTON – US prisons are becoming a hotbed for indoctrinating inmates and turning them into radical Muslims, US lawmakers were told Wednesday in the second of a series of controversial hearings.

"Despite appearances, prison walls are porous. Outside influences access those on the inside, and inmates reach from the inside out," Patrick Dunleavy, a retired New York prison inspector, told US lawmakers.

"Individuals and groups that subscribe to radical Islamic ideology have made sustained efforts to target inmates for indoctrination."

He was addressing a hearing of the House Homeland Security Committee, chaired by Representative Peter King, whose first session in March on Muslim radicalization in the United States drew accusations of a religious witch hunt.

"Dozens of ex-cons who became radicalized Muslims inside US prisons have gone to Yemen to join an Al-Qaeda group run by a fellow American, Anwar al-Awlaqi," King, a Republican, told the committee.

Awlaqi's "terrorists have attacked the US Homeland several times since 2008 and are generally acknowledged to be Al-Qaeda most dangerous affiliate," he added.

King has accused Muslim leaders and mosque imams of doing too little to stop the radicalization of young Americans and are not cooperating with law enforcement.

The United States has the highest incarceration rate in the world with about 1.4 million people behind bars across the country, meaning there is potentially a captive audience.

"The prison population is vulnerable to radicalization by the same agents responsible for radicalizing Americans outside of the prison walls," said Dunleavy, who has investigated terror recruitment in New York prisons.

"The initial exposure to extremist jihadi Islam may begin in prison. However, it often matures and deepens after the release."

And militant groups have little difficulty in smuggling materials into cells, he charged.

"Jihadi and extremist literature finds its way through the mail, even though it is largely prohibited. Anything can be gotten in prison, including a PDA or a smartphone," he said.

But Purdue University sociology professor Bert Useem was less alarmist, saying: "The crux of my testimony is that prisons have not served as a major source of jihad radicalization."

He insisted prison guards were vigilant about the dangers. "Rather than waiting for facilities to be penetrated by radicalizing groups, correctional leaders have fashioned, staffed and energized the effort to defeat radicalization," he said.

And former assistant US attorney Kevin Smith said: "The particular group that we're talking about, these particular radicalized inmates, represent a very small proportion."

Although he recognized that "it's a small portion with a much greater exponential danger for the community."

And Democratic Representative Hansen Clarke, who grew up in the tough city of Detroit, grew emotional when he talked about friends who had spent years in prison, blaming radicalization on a failure of the prison system.

He said he had asked someone why people converted to Islam in jail.

"You know, essentially, it's two reasons: number one, for protection -- to protect myself from other inmates and the prison staff. And then, number two, because these young men were tired of their past. They wanted to break away from their criminal past and to become a new man. So they became Muslim.

"These young men are going to Islam. They're trying to protect themselves. They want to change themselves. Are there some bad folks? Yes, there are, like in every other faith and every other organization."