Hours earlier, on Wednesday, a criminal court in Kuwait city ordered the release on bail of Nasir al-Mutairi whom the United States captured in Afghanistan in 2001 and detained at Guantanamo Bay, and returned to Kuwait in January.
His lawyer, Mubarak al-Shimmiri, said the judge had ordered al-Mutairi's release after discussions in chambers following a hearing in his trial.
Bail was fixed at 200 Kuwaiti dinars ($680) and al-Mutairi is not allowed to leave Kuwait.
"We cannot believe our brother is home," al-Mutairi's eldest sister, Um Yusuf, said in a phone call. "We thank God for accepting our prayers, and bringing him back."
She said al-Mutairi was too tired to speak to the media on Wednesday. Another person who saw him, Khalid al-Odah, said al-Mutairi "looked very happy, but he quickly tired".
The al-Mutairi family campaigned
for Nasir's release from detention
Al-Odah heads a non-government group that campaigns for the release of Kuwaiti detainees at the US naval base in Guantanamo Bay.
Al-Mutairi is so far the only Kuwaiti to return to his home country from Guantanamo.
US soldiers detained al-Mutairi in Afghanistan in December 2001 after American-backed forces had overthrown the Taliban government.
He spent much of his time in US custody at Guantanamo and was flown to Kuwait in January this year and detained on arrival.
He is now standing trial on charges of harming Kuwait by working for a foreign country, joining foreign military forces without permission, and undergoing illegal weapons training.
"We cannot believe our brother is home. We thank God for accepting our prayers, and bringing him back"
al-Mutairi's eldest sister
The charges do not mention terrorism or name any particular country or force. Al-Mutairi has pleaded not guilty.
The court also ordered the Guantanamo Bay interrogation records of al-Mutairi be made available to the defence, lawyer al-Shimmiri said. The US has handed the records to Kuwait.
The defence argues that al-Mutairi is not a "militant", and had been working in Afghanistan to help those displaced and impoverished by the country's long-running civil war.
The trial was adjourned to 1 June.
Eleven other Kuwaiti suspects are still detained in Guantanamo Bay, along with hundreds of others from more than 40 countries.