Steinmeier, who had served as former chancellor Gerhard Schröder's chief of staff until he left office in November, insisted that the center-left government had worked to secure the liberation of Murat Kurnaz, now 24.

"There were already efforts made under the Red-Green government to win Kurnaz's freedom," he told reporters on the sidelines of a Social Democratic Party event, referring to the previous Socialist-Green administration.

Kurnaz's lawyers allege that the Schröder government received an offer by the US administration in 2002, just months after Kurnaz's arrest in Pakistan and transfer to Guantanamo, to return him to Germany but refused due to security concerns.

Calls for investigation

The liberal opposition Free Democratic Party (FDP) accused the former government of negligence. It called over the weekend for a parliamentary investigation of the case following Kurnaz's release and return to Germany, where he is a permanent resident, on Thursday.

"The question must be answered as to whether the government at the time failed to insist sufficiently on Kurnaz's release out of consideration for the US in terms of foreign policy," FDP interior affairs expert Max Stadler told the daily Berliner Zeitung.

He said Kurnaz should appear as a witness before an existing parliamentary committee probing alleged abuses by the foreign intelligence service in the international fight against terrorism.

Kurnaz, nicknamed the "Taliban of Bremen" for the northern German city where he had lived, was captured in Pakistan after the Sept. 11, 2001 attacks in the United States on suspicion of terrorist activities and then taken to Guantanamo Bay, Cuba in 2002.

His lawyers say he was subjected to torture and sexual humiliation at the lock-up in Cuba.

Criticism of the camp grew louder in Germany with the government's human rights spokesman, Günter Nooke, repeating Chancellor Angela Merkel's call for its closure.

"Kurnaz is free but the fundamental problem remains," Nooke told the Berliner Zeitung.

Around 450 prisoners are still being held at Guantanamo, many of them picked up as suspected al Qaeda or Taliban fighters from Afghanistan's battlefields.