US President Joe Biden says the US is certain the Syrian government is imprisoning American journalist Austin Tice, who went missing in the war-torn country a decade ago.
"We know with certainty that he has been held by the Syrian regime," Mr Biden said in a statement released by the White House to mark the 10th anniversary of Tice's abduction.
"We have repeatedly asked the government of Syria to work with us so that we can bring Austin home."
Syrian President Bashar al-Assad's government has never publicly acknowledged it is detaining him.
But Mr Biden's statement was the clearest indication yet that the US was certain Mr Tice was being held in a Syrian government prison.
Mr Tice, a freelance journalist and former marine, went missing shortly after his 31st birthday on August 14, 2012, at a checkpoint in a contested area west of the capital Damascus.
A video released a month later showed him blindfolded and held by armed men. He has not been heard from since.
"On the 10th anniversary of his abduction, I am calling on Syria to end this and help us bring him home," Mr Biden said.
He added there was "no higher priority in my administration than the recovery and return of Americans held hostage or wrongfully detained abroad".
Another American, psychologist Majd Kamalmaz, was detained at a checkpoint in Damascus in February 2017. He has not been heard from since.
US basketball star Brittney Griner and former US marine Paul Whelan are both being held in Russia, and the Biden administration says it has issued a "serious offer" to Russia for a prisoner swap to secure their release.
'Impossibly brutal things'
Another American, Kevin Dawes, now 39, was also captured in Syria in 2012 and held for three and half years before he was released back to the US with the help of Russia.
Mr Dawes described horrific torture and abuse he and other prisoners suffered during his detention.
"Austin [Tice] has endured impossibly brutal things and witnessed crimes against other human beings not seen since the Nazi holocaust. It is a miracle that he is alive," he told the ABC.
"If he ever again walks on American soil, I will gift him the resources that I have cultivated."
The Syrian government did not acknowledge it was holding Mr Dawes until another prisoner, British orthopaedic surgeon Abbas Khan, got word out via his family.
Mr Dawes believes Mr Khan, who was later executed, saved his life.
While he never directly encountered Mr Tice during his imprisonment, Mr Dawes said he had no doubt he was still being held by the Syrian government.
Four years ago, then-US envoy to Syria, James Jeffrey, said Mr Tice was believed to be alive and held hostage in Syria. He did not say why officials believed this or who might be holding him.
In May, top Lebanese security official Major General Abbas Ibrahim met with US officials in Washington as part of mediation efforts between the US and Syria for Mr Tice's release.
Mr Ibrahim, the chief of Lebanon's General Security Directorate, has mediated complicated hostage releases in the past.
In the final months of the Trump administration, two US officials — including the government's top hostage negotiator — made a secret visit to Damascus to seek information on Mr Tice and other Americans who had disappeared in Syria.
It was the highest-level talk in years between the US and Mr Assad's government, though Syrian officials offered no meaningful information on Mr Tice.
In May, Mr Biden met Mr Tice's parents in the oval office and reiterated his commitment to continue to work through all available avenues to secure "Austin's long overdue return to his family".
"We are anxious to see the President's uplifting words put into action, confirming the assurance that Austin's safe return is a priority for this administration," Marc and Debra Tice said in a statement following the meeting.
"With renewed vigour, we will continue to push for the diplomatic engagement and action necessary to see Austin walk free. May it be soon."
Mr Tice was published by The Washington Post, McClatchy newspapers and other outlets.
He went to Syria to cover the conflict which started in 2011 and has since left hundreds of thousands dead and nearly half the pre-war population of 23 million displaced.
Mr Biden signed an executive order last month aimed at deterring and punishing those responsible for the wrongful detention of Americans abroad by authorising government agencies to impose sanctions and other measures.
