(CNN) -- Three American college students were being held Tuesday, possibly in a Cairo courthouse, accused of lobbing Molotov cocktails during the unrest gripping Tahrir Square, authorities and family members said.
The news came as a blow to their families in the United States, especially after seeing a video aired on Egyptian state television showing the three students nervously standing shoulder to shoulder along a wall in front of what appeared to be liquid-filled bottles.
"I can't remember seeing his face look like that," said Nicole Sweeney, whose brother Derrik Sweeney is among those detained. "He looks absolutely terrified."
American University in Cairo identified the three detained students as Sweeney, 19, a Georgetown University student from Jefferson City, Missouri; Gregory Porter, 19, from Glenside, Pennsylvania, who attends Drexel University in Philadelphia; and Luke Gates, 21, of Bloomington, Indiana, who goes to Indiana University.
They were spending the semester in Cairo as part of a study-abroad program, according to American University.
Cairo police have questioned the three, who were detained Monday night, and they will face additional investigation by prosecutors, said Adel Saeed, a spokesman for the office of Egypt's general prosecutor.
"The three boys were throwing Molotov cocktails and had no passports on them when they were picked up," Saeed said.
Later, he told CNN that the prosecutor was unable to question the students on Tuesday because of ongoing protests. They will be questioned on Wednesday in the presence of legal representatives from the U.S. Embassy, he said.
"They were throwing explosives at security forces -- which is a crime," Saeed said.
Nicole Sweeney said her family had been told the men were apparently being held in a courthouse.
State Department spokeswoman Victoria Nuland did not comment on the men's whereabouts during a briefing with reporters, but said officials believe they know where the three are and that they are safe.
"I don't think we have any information to indicate otherwise," she said, adding that she did not believe the men had been formally charged.
Social-media posts appear to show Gates and Sweeney in the thick of recent protests in Cairo, with Gates indicating he had been injured in clashes over the weekend.
"Earlier tonight rubber bullets a charge and then a retreat," said a message posted Sunday from a Twitter account with Gates' name and a photo resembling one of the men in the police video. The poster added that his knee and elbow were hurt.
On Saturday, the writer said that "we were throwing rocks and one guy accidentally threw his phone."
Another Facebook account shows a man resembling Sweeney during protests in Cairo.
CNN could not independently confirm the authenticity of the posts.
Gates' father, George William Gates, told CNN that he learned of his son's detention in a call from an American University official.
"It was a hard call to get," he said.
"He told us that everything was fine and he was really happy," Gates said of an earlier conversation with his son. "Luke said so many people were friendly to him and that it was easy to make friends there."
Sweeney's mother, Joy Sweeney, said she was worried about her son's safety, but he had assured her he was fine the last time they spoke.
"He has always been somebody who believes in democracy and standing up for what you believe in and I know that he was excited at the opportunity to be in Egypt right now. I also know that he is an extreme pacifist," she told CNN's Brooke Baldwin. "He would never do anything to inflict violence on somebody else."
News of their detention came on the fourth consecutive day of clashes between protesters and police in and around Tahrir Square, which also was the focal point of a popular uprising that ousted Hosni Mubarak as president in February. Demonstrators are calling for the removal of the Supreme Council of Armed Forces, the ruling body that took power after Mubarak's ouster.
Thirty people have died in the clashes since Saturday, according to Egypt's Health Ministry. About 1,950 have been injured.
Georgetown and Drexel officials both released statements saying they are working with American University and U.S. authorities to obtain their students' release.
David Lynfield, deputy press officer for the U.S. Embassy in Cairo, said the Egyptian authorities' "claims are being currently investigated."
Sweeney interned for U.S. Rep. Blaine Luetkemeyer, R-Missouri, between February and May, press secretary Pat Sloca said. Luetkemeyer has been in touch with the U.S. State Department about Sweeney's detention, said Sloca, who added that the student had been an "outstanding worker" during his internship.
He is a dual Arabic and psychology major who wanted to get more experience in an Arabic-speaking country, according to his sister.
"The Cairo program was actually the most stable of his options," she said.
Amin Bonnah, a visiting professor at Georgetown who taught an Arabic class Sweeney took, recalled him as a "very peaceful, socially peaceful, loving person."
"I do not believe he is the kind of person who did that," Bonnah said.
Gates is a junior who is pursuing a double major in political science and Near Eastern languages and culture, said Ryan Piurek, communications director at Indiana University.
Gates has been interested in different cultures since childhood and began applying to overseas programs before the Egyptian revolution this year, according to his father.
Porter is an international studies major at Drexel, according to university spokeswoman Niki Gianakaris.
The American University in Cairo, where the three were studying, describes itself as "an American liberal arts university in the heart of the Middle East." The university was founded in 1919 by Americans and says it "offers a vital bridge between the cultures of East and West." It is chartered and accredited in both the United States and Egypt.
The university boasts nearly 5,000 undergraduates from 113 countries as well as the largest English-language library in Egypt. The AUC Press is the leading publisher of English-language books in the Arab world, the school says.
CNN's Ashley Hayes, Ashley Fantz, Jamie Crawford, Brian Todd, Dugald McConnell, Janelle Griffin and journalist Mohamed Fadel Fahmy contributed to this report.