The alleged mastermind of the 9/11 attacks and four suspected co-plotters will be tried in a civilian court blocks from where Al-Qaeda hijackers crashed two airliners into the World Trade Center, the US government announced.

Attorney General Eric Holder said Friday that prosecutors would seek the death penalty against Khalid Sheikh Mohammed and four other suspects who are held at Guantanamo Bay but will be moved to a New York prison ahead of their trial.

"After eight years of delay, those allegedly responsible for the attacks of September 11 will finally face justice," Holder said, without giving a date.

"They will be brought to New York to answer to their alleged crimes in a courthouse just blocks away from where the Twin Towers once stood."

Five more Guantanamo detainees, including Abd al-Rahim al-Nashiri, accused of plotting the 2000 bombing of the USS Cole destroyer off Yemen that killed 17 US sailors, will be tried before military commissions.

The military tribunals were heavily criticized after being set up by former president George W. Bush in late 2001, but have since been reformed to grant defendants more rights to evidence and bar evidence obtained through torture.

The announcement, key to President Barack Obama's plans to shutter Guantanamo by January, was blasted by families of the nearly 3,000 victims of the September 11, 2001 attacks.Related article: Families blast trial

"To allow a terrorist and a war criminal the opportunity of having US constitutional protections is a wrong thing to do and it's never been done before," said Ed Kowalski of the 9/11 Families for a Secure America Foundation.

Peter Gadiel, who lost his 23-year-old son James in the World Trade Center's north tower, accused Obama of trying to establish a "show trial" that would end up being "a circus."

The decision drew flak from Obama's Republican foes in Congress, who have mounted a vigorous campaign to block the transfer of Guantanamo detainees to US soil.

Senate Republican Minority Leader Mitch McConnell called it "a step backwards for the security of our country" that "puts Americans unnecessarily at risk."

The city's mayor, Michael Bloomberg, said he supported holding the trial in New York, which suffered the brunt of the attacks.Related article: Suspects face angry New York

"It is fitting that 9/11 suspects face justice near the World Trade Center site where so many New Yorkers were murdered," Bloomberg said.

Meanwhile, the American Civil Liberties Union hailed the move.

"The transfer of cases to federal court is a huge victory for restoring due process and the rule of law, as well as repairing America's international standing, an essential part of ensuring our national security," the group's executive director, Anthony Romero said.

"We can now finally achieve the real and reliable justice that Americans deserve. It would have been an enormous blow to American values if we had tried these defendants in a process riddled with legal problems," he said.

The move to a civilian trial signaled a major shift in the treatment of "war on terror" suspects and raised serious legal questions about evidence potentially tainted by harsh interrogation techniques.

Sheikh Mohammed is known to have been waterboarded -- subjected to simulated drowning -- 183 times during his years in US custody.

Holder, citing information not yet made public, asserted the tainted evidence would not prevent a "successful" outcome of the trials.

He insisted that a New York jury could still be impartial and that all legal requirements would be met before the suspects are brought onto US soil, with Congress being given a 45-day warning.

Defense Secretary Robert Gates hailed the move as a major step forward as Obama seeks to close the detention center at the US naval base at Guantanamo Bay, Cuba by his self-imposed January 22 deadline.

Officials told AFP that up to 65 of the 215 inmates who still linger at Guantanamo are ready for trial and Holder said to expect more announcements in "the very near future."

Another 69 Guantanamo inmates are cleared for release but struggling to find countries to take them in. The fate of the remainder -- less than 100 -- remains unclear.Profile of 9/11 suspects

Greg Craig, tasked by the White House with overseeing Guantanamo's closure, resigned Friday after criticism of his handling of the process.

Earlier military commission charges against Sheikh Mohammed and co-defendants Ramzi Binalshibh, Ali Abd al-Aziz Ali, Walid bin Attash and Mustapha al-Hawsawi were suspended when Obama launched his policy review.

In addition to the September 11 attacks, Sheikh Mohammed has claimed some 30 operations against the West, including the 2002 beheading of US journalist Daniel Pearl in Pakistan.

After his March 1, 2003 capture, the Pakistani man self-proclaimed as the mastermind of the 9/11 attacks was handed over to US agents who held him in secret prisons for over three years before sending him to Guantanamo in September 2006.