The Obama administration Monday abandoned its effort to put Khalid Sheikh Mohammed and four other alleged plotters of the Sept. 11, 2001, attacks on trial in civilian courts, saying the accused terrorists will instead face military trials at Guantanamo Bay, in the prison President Barack Obama had promised would be closed more than a year ago.

The move, announced by Attorney General Eric Holder on the same day that Mr. Obama formally launched his bid for re-election, marked a significant reversal on an issue that has vexed two successive administrations.

Mr. Holder blamed politics for the continued delay in bringing the defendants to justice.

"We were prepared to bring a powerful case against the 9/11 defendants in federal court," he said, visibly frustrated and striking a defiant tone. "Unfortunately, members of Congress have intervened and imposed restrictions blocking the administration from bringing any Guantanamo detainees to trial in the United States."

Mr. Holder also said he shared the frustration of family members of 9/11 victims. "We simply cannot allow a trial to be delayed any longer for the victims of the 9/11 attacks or for their family members who have waited for nearly a decade for justice," he said.