The US Supreme Court has rejected an appeal by Jose Padilla, who was held until recently as an "enemy combatant".
Jose Padilla has been moved to civilian custody in Miami
In a 6-3 vote, the court sided with the Bush administration by refusing to rule on whether US citizens suspected of terrorism could be held without charge.
Mr Padilla was moved to civilian custody in January after being held for more than three years without charge.
Government lawyers argued that the Supreme Court appeal was no longer relevant in the light of that move.
Mr Padilla's case is now being dealt with by a civilian court in Miami, Florida.
In November 2005 he was charged with plotting to "murder, kidnap and maim" abroad.
The charges do not include allegations made against him when he was arrested in May 2002, of plotting to set off a radioactive device or "dirty bomb" in a US city.
Mr Padilla, a former Chicago gang member and a convert to Islam, was arrested following a trip to Pakistan.
His military detention was heavily criticised by civil rights groups and the case was seen as a test of the limits of the US government's anti-terrorism powers.
The three judges who voted in favour of allowing Mr Padilla to appeal were David H Souter, Ruth Bader Ginsburg and Stephen Breyer.
The Supreme Court ruling fell one vote short of the four needed to grant an appeal.