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Texas Rep. Ron Paul, who is running for the Republican presidential nomination, cried foul Friday over the killing of a radical U.S.-born cleric in Yemen without a trial, joining a chorus of civil liberties groups raising "due process" concerns over the drone attack.

"No one likes these kind of people, but I also like the rule of law and I like our Constitution, that you don't just target people, assassinate them, someone who has not been charged and you have no proof of anything," Paul told Fox News. "So if we want to protect American citizens from that type of justice, we have to be more cautious."

U.S. counterterrorism forces not only killed Anwar al-Awlaki, an Al Qaeda leader, early Friday but also three other operatives, including American-born Samir Kahn, who edited a Jihadi Internet magazine. Al-Awlaki's reputation as an instigator of terrorist plots had grown in recent years, especially in the power void created in Al Qaeda with the killing of Usama bin Laden earlier this year.

White House Press Secretary Jay Carney refused to answer questions Friday about the legality of the government targeting and killing an American citizen without a trial.

"I'm not going to address the circumstances of Awlaki's death," he said.

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John Ashcroft, who was the attorney general in the Bush administration, told Fox News that he believes the operation was legal.

"The president goes through a significant process before he signs an order regarding these kinds of things," he said. "It would be based on evidence presented to him from the intelligence community, and I would expect that this evidence was not just of statements by al-Awlaki but that he was involved operationally not only in sighting but in carrying out terrorist attacks."

But Rep. Dennis Kucinich, D-Ohio, joined Rep. Paul in condemning the killing of al-Awlaki, saying the Obama administration had "crossed a dangerous divide and set a dangerous precedent for how the United States handle terrorism cases."

"Mr. al-Awlaki's allegedly violent rejection of America was not acceptable in any way," Kucinich said. "Neither is it acceptable to trample the Constitution through extrajudicial killings."

The majority of lawmakers on Capitol Hill and the leading Republican presidential contenders -- Texas Gov. Rick Perry and former Massachusetts Gov. Mitt Romney -- praised the killing, and Obama touted the deadly attack as a "major blow to Al Qaeda."

Those sentiments weren't shared by Rep. Paul, known for his libertarian stances. However, when asked about the legality of killing of Usama bin Laden, he said that was different.

"The authority was given to go after those who participated in the planning and the carrying out of 9/11, and that is not what al-Awlaki's charges are, and he wasn't involved at all," he said.

The largest Muslim advocacy group in the nation responded to news of Awlaki's death by rejecting his calls to violence, though the group also raising the "due process" concerns that were amplified by other civil liberties groups.

The Council on American-Islamic Relations, or CAIR, said the American Muslim community "firmly repudiated Anwar al-Awlaki's incitement to violence, which occurred after he left the United States."

"While a voice of hate has been eliminated, we urge our nation's leaders to address the constitutional issues raised by the assassination of American citizens without due process of the law," the controversial group said.

CAIR's carefully worded statement reflects the group's attempt to balance its advocacy on behalf of American Muslims targeted for discrimination and hate crimes since the Sept. 11 terrorist attacks with its condemnation of the radical ideology that fuels the terror.

CAIR itself has come under fire for being named by the Justice Department as an unindicted co-conspirator in a major terror financing case involving the Holy Land Foundation.

Other groups were even more direct in their criticism of the killing of al-Awlaki in a U.S. drone attack in Yemen.

"The targeted killing program violated both U.S. and international law," ACLU Deputy Legal Director Jameel Jaffer said in a written statement. "As we've seen today, this is a program under which American citizens far from any battlefield can be executed by their own government without judicial process, and on the basis of standards and evidence that are kept secret not just from the public but from the courts."

The Center for Constitutional Rights, which had previously brought a challenge in federal court to the legality of the authorization to target al-Awlaki in Yemen, called the assassination by drone attacks "the latest of many affronts to domestic and international law."

"The targeted assassination program that started under President Bush and expanded under the Obama administration essentially grants the executive the power to kill any U.S. citizen deemed a threat, without any judicial oversight, or any of the rights afforded by our Constitution," said Vince Warren, executive director of the center.

"If we allow such gross overreaches of power to continue, we are setting the stage for increasing erosions of civil liberties and the rule of law," he said.