WASHINGTON (Reuters) - The U.S. military judge in the case of the accused mastermind of the September 11 attacks has decided to retire rather than continue to oversee the complex proceedings, defense officials said on Monday.

Marine Col. Ralph Kohlmann, chief judge for the U.S. Military Commission trying detainees accused of terrorism in the Guantanamo naval base, will retire five months earlier than expected, officials said. On Monday, he named U.S. Army Col. Steven Henley as his replacement.

A defense official said Kohlmann had planned to retire in April but decided to do it now after concluding that the proceedings would likely continue beyond next spring.

Khalid Sheikh Mohammed, the accused al Qaeda planner of the 2001 attacks on New York and Washington, faces a military commission trial with four co-defendants at the U.S. naval base at Guantanamo Bay, Cuba, where they are being held.

Mohammed and his co-defendants -- Ramzi Binalshibh, Mustafa Ahmed al Hawsawi, Walid bin Attash and Ali Abdul Aziz Ali -- are charged with conspiracy and 2,973 counts of murder representing all of those killed when hijacked airliners crashed into New York’s World Trade Center, the Pentagon and a Pennsylvania field.

President-elect Barack Obama, who enters the White House on January 20, has vowed to shut the U.S. prison at Guantanamo.

The American Civil Liberties Union, which has long called for the shuttering of the jail, suggested in a statement on Monday that Kohlmann’s departure could be a consequence of Obama’s plans.

“The timing of the announcement to replace the military commission judge on the 9/11 cases is highly suspect and disturbing,” ACLU executive director Anthony Romero said.

He said Bush administration could try to “sabotage” Obama’s plans by “ramming through these cases in its last days.”

But a second defense official said there was nothing suspicious about Kohlmann’s departure, nothing that his retirement plans were discussed in open court in September.

Hearings are scheduled to resume in the case on December 8.