A former New York prisons official testified Wednesday that radical Muslims have made "sustained efforts" to indoctrinate inmates in America, at the second hearing on Islamic radicalization held by Rep. Peter King.

The first round of King's hearings on domestic threats in March drew passionate protests, and the hearing Wednesday on radicalization in U.S. prisons drew similar objections. Rep. Bennie Thompson, D-Miss., the top Democrat on the House Homeland Security Committee, said at the opening of the hearing that the threat of terrorism from U.S. converts in prison is "small."

"There are other threats to be concerned about," Thompson said.

Rep. Laura Richardson, D-Calif., suggested the hearing was "racist," asking why Muslims in prisons are being targeted as opposed to other religious or ethnic minorities.

But law enforcement officials called to testify said Islamic radicalization in the prison system is a real threat.

Patrick Dunleavy, a retired official in the New York State Department of Correctional Services, said radical Muslims have been trying to convert U.S. inmates to their cause for decades.

"Despite appearances, prison walls are porous," he said. "Individuals and groups that subscribe to radical Islamic ideology have made sustained efforts to target inmates for indoctrination."

Kevin Smith, former federal prosecutor in California, cited the case of Kevin James and Levar Washington, who pleaded guilty in 2007 to "conspiracy to levy war against the United States through terrorism."

Smith called it a "seditious conspiracy" hatched inside California's prison system.

Michael Downing, a top official in the Los Angeles Police Department, described the conversions as a "phenomena of low volume," but one that holds "high consequence" considering the sheer size of the U.S. prison population.

"We do have a problem," he said. "Prisons are communities at risk."

King, R-N.Y., claims the prisons are a hotbed of radical conversion, but that he's not targeting Muslims.

"What I'm targeting is violent terrorists and extremists, and ... the fact is that Al Qaeda itself has said it will recruit within the Muslim-American community. That's the reality," he told Fox News.

Members of several New York organizations that protested the first hearings were back for the second round, saying the hearings play on stereotypes of both Muslims and prisoners. A coalition of civil rights, religious and interfaith groups met on New York's Long Island Tuesday.

The protesters included an imam who works as a chaplain in a county jail on Long Island. He says he has seen no evidence of terrorist recruitment at the jail.

"If we found anyone in our community committing an act of terrorism, by the time the police got there the matter would be settled and there would be one less terrorist," he said.

Since the Sept. 11 terrorist attacks, there have been at least four cases of American Muslims being radicalized in prison.

Bert Useem, a professor at Purdue University, pointed to the relatively low number of confirmed cases in claiming Wednesday that the threat of prison radicalization is relatively low.

"Prisons have not served as a major source of Jihad radicalization," Useem said.

King said that the Obama administration has been working with him to address the issue, noting that Homeland Security Secretary Janet Napolitano is setting up a task force to look into radicalization in the prisons.

King said the next hearing after Wednesday's will likely be held in late July and will focus on reports of Americans joining Al Qaeda in the Arabian Peninsula, a Yemen-based offshoot of Al Qaeda that has been linked to attempted attacks on U.S. targets, including the foiled Christmas 2009 bombing of an airliner over Detroit and explosives-laden parcels found on cargo flights last year.

The Associated Press contributed to this report.