Rep. Peter King, the new chairman of the Homeland Security Committee who has ignited a storm of controversy for planning hearings next month on the growing threat of radical Islam, is refusing to bow to pressure to broaden the scope of the probe to include extreme environmentalists and white supremacists.

King told FoxNews.com that he plans to hold the first of a series of hearings over the next year and half in the first or second week of March. Muslim groups, their supporters and the top Democrat on the Homeland Security Committee fear that hearings focused solely on radical Islam will devolve into an anti-Muslim witch hunt.

But King told Rep. Bennie Thompson, D-Mo., in a letter Tuesday that the committee "will continue to examine the threat of Islamic radicalization, and I will not allow political correctness to obscure a real and dangerous threat to the safety and security of the citizens of the United States."

King has said that law enforcement officials around the country have told him they receive little cooperation from Muslims in trying to foil terror plots.

Fifty-one Muslim, civil rights and interfaith groups, including the Council on American-Islamic Relations (CAIR), Amnesty International USA, and the Interfaith Alliance, wrote to House Speaker John Boehner and House Democratic leader Nancy Pelosi last week, comparing the planned hearings to "McCarthyism."

"We strongly urge you to object to the hearings in their current form. If Chairman King wishes to address violent extremism, then we hope you will ensure that he examines violence motivated by extremist beliefs, in all its forms, in a full, fair and objective way," they added. "The hearings should proceed from a clear understanding that individuals are responsible for their actions, not entire communities."

But King said broadening the scope would dilute the hearings.

"In short, the homeland has become a major front in the war with Islamic terrorism and it is our responsibility to fully examine this significant change in Al Qaeda tactics and strategy," he said in the letter. "To include other groups such as neo-Nazis and extreme environmentalists in this hearing would be extraneous and diffuse its efficacy. It would also send the false message that our committee believes there is any threat equivalency between these disparate groups and Islamist terrorism."

King told The New York Times on Monday that he would rely on Muslims to bolster his argument that American Muslim leaders have failed to cooperate with law enforcement officials to foil terrorist plots. He told the newspaper he did not plan to call any of the local law enforcement or counterintelligence experts who he said had told him repeatedly that noncooperation by American Muslims is a "significant issue" – a claim he says they will only state privately.

CAIR is now questioning why King won't invite any authorities to appear at the hearing.

"Rep. King seems to believe that he need not offer any evidence or expert testimony to back up his baseless allegations," CAIR legislative director Corey Saylor said in a statement. "One wonders whether Rep. King will call witnesses to support his bizarre claim that '85 percent' of American Muslim community leaders are 'an enemy living amongst us.'"

But King explained to FoxNews.com that what he told The New York Times is that it's "difficult to get law enforcement." But he added he will have law enforcement officials testify even if they're retired.

"There will be law enforcement familiar with the facts," he said. "These people will be actively involved in counterterrorism."

King noted that federal prosecutors have named CAIR as an unindicted co-conspirator in a plot to finance the terrorist group Hamas – a status that CAIR strenuously objects to.

And King said he based his 85 percent claim on the testimony of Muslim leader Sheikh Kabbani, who told the State Department in 1999 that he thought over 80 percent of the mosques in the U.S. are controlled by radical imams.

Asked about fears that his hearings would become an example of modern day McCarthyism, King said, "This is absolutely ridiculous." Pointing to the threat of homegrown terrorism that Attorney General Eric Holder said last month keeps him awake at night, King said, "it would be irresponsible of me not to investigate that."