Nizar Zakka, who was detained in Iran on spying charges between 2015 and 2019, now runs Hostage Aid Worldwide, a non-profit group working with families to free kidnapped civilians.
This week he was in Syria in the immediate aftermath of the overthrow of Assad.
Both Zakka's group and the US government believe Assad or his former officials are holding Tice, a 43-year-old reporter who was kidnapped in Syria in August 2012.
The overthrow of the Assad clan has allowed thousands of Syrian prisoners -- and one other American -- to escape Syria's notorious detention centres, but many are still missing, including Tice.
Tice was working for Agence France-Presse, McClatchy News, The Washington Post, CBS and other media outlets when he was detained at a checkpoint in Daraya, a suburb of Damascus.
Zakka, who is in contact with Syria's new transitional government led by the Islamist rebel fighters of Hayat Tahrir al-Sham, has been scouring liberated prisons for signs of the missing American.
Helicopter escape
But he believes that members of the ousted Assad clan or former officials may still be holding him, hoping to use his fate as leverage to secure a ransom or some kind of legal protection.
Before the fall of Assad, Zakka had a good idea of where Tice might be.
"The intel we have as late as early January 2024 was that he was in certain places," the Lebanese-American campaigner told AFP in Damascus.
"We have been tracking all along. We have intel about where he was at this date, where he was at that date, which prison, which guard."
But after the rapid collapse of Assad's rule threw the vast Syrian detention network into chaos, this data could only lead Hostage Aid so far.
Zakka thinks a senior former government figure such as Maher al-Assad, the leader's younger brother, may have arranged to hide Tice inside Syria.
A former official of the ousted government told AFP that Maher only learned about his brother's escape after the fact and subsequently took a helicopter, probably towards the Iraqi capital Baghdad.
Text message campaign
Zakka favours a theory that Tice is being held in a secret safehouse somewhere in or near Damascus.
"Because I don't believe the regime will trust anybody to take him," he explained. "They cannot take him to Russia with them because I'm sure the Russian government would not accept that.
"And I don't believe they're going to trust the Iranians ... So I believe he's somewhere over here."
Hostage Aid is therefore pushing on with its campaign to find Tice inside Syria, sending a million text messages to citizens asking for information and broadcasting televised appeals for information.
The drive has already borne unexpected fruit. Civilians who found an escaped American detainee that the US government did not even know was in Syria contacted the Tice hotline immediately.
Travis Timmerman, 29, had been reported missing in Budapest, Hungary, but was found wandering shoeless in the Damascus suburbs after the doors of the notorious "Palestine Branch" detention centre were thrown open.
Thanks to the Tice campaign, his Syrian rescuers knew who to call -- giving Zakka hope that the other US hostage may also soon be freed.
"We never failed, and this time we're not going to fail and I promised the mom that we're going to get Austin home," Zakka declared.
