(CNN) -- The mother of an Ohio-born man is "hanging on by a thread" after her son disappeared earlier this month in violence-wracked Syria, according to family members.
Obada Mzaik, a dual American and Syrian citizen, went missing on January 3 after he traveled with his brother on a flight from Detroit to Damascus, his uncle, Dr. Firas Nashef, told CNN.
Mzaik was studying civil engineering at a private university in the Syrian capital and had planned to pursue a master's degree in the United States, his uncle said.
The 21-year-old student was born in Columbus, Ohio, but moved with his family back to Syria when he was child. He had enrolled in a seasonal program at a Michigan community college.
His uncle said Mzaik returned to Syria earlier this month with his brother, Obaie Mzaik, to visit family and prepare for the start of the new semester in Damascus.
The 19-year-old brother said that upon their arrival, they were detained by security officers for questioning, according to Nashef.
The younger brother was soon released, but the officers kept Obada Mzaik for further questioning.
"In his mind, he was expecting to see his brother in the morning," said Nashef. "But that never happened."
Mzaik's family is now fearing the worst.
"We hear some horrendous stories from over there," said Nashef.
"And my sister is hanging on by a thread," he said of the missing student's mother. "We're all hanging on with a bit of hope. But she's angry and she has no information."
This month's arrest isn't the first time the 21-year-old has been detained by Syrian security forces.
Last summer, he was arrested and held for 37 days for "spreading fliers about the revolution," according to his uncle.
A Facebook page, meanwhile, is calling for Mzaik's release.
The U.S. State Department also weighed in Tuesday, saying it's aware of reports of his disappearance.
"We are working with local authorities to ascertain (Mzaik's) welfare and whereabouts," said State Department spokesman Noel Clay.
Mzaik is at least the second U.S. citizen to have disappeared this month in Syria, raising questions as to whether the missing men could usher in a more public State Department presence in the restive country.
Abdelkader Chaar, 22, is thought to have been arrested in Aleppo, Syria, five days after Mzaik disappeared.
Chaar was born in Syracuse, New York, moved to Aleppo with his parents when he was a boy and is a medical student at Aleppo University, his uncle said. His family has not been told why he was arrested, said Sam Chaar, who spoke to CNN from Arizona.
The disappearances of both men come as anti-government demonstrations raged throughout the country.
Protesters last week focused their attention on political prisoners and demanded the release of detainees. At least 10 people were slain in clashes Friday, according to the Local Coordination Committees of Syria, an opposition activist group.
For more than 10 months, Syria has been in the throes of an anti-government public uprising and a brutal security crackdown against protesters. The United Nations last month estimated more than 5,000 deaths since mid-March. Opposition groups estimate more than 6,000 people have died.
Also, the Arab League is extending its monitoring mission to see whether the government is adhering to an agreement to end the violence.
The Arab League has called on President Bashar al-Assad's regime to stop violence against civilians, free political detainees, remove tanks and weapons from cities and allow outsiders, including the international news media, to travel freely around Syria.