The sister of one of three US students arrested in Cairo and shown on state television said he was being held in an Egyptian courthouse, which she hoped was a good sign.

Nicole Sweeney told the Guardian that her parents knew very little, but had been told that 19-year-old Derrik Sweeney, who was arrested on Monday, was not being detained in prison.

Derrik, a Georgetown University student, is one of three US students who have been accused of throwing petrol bombs at security forces in Tahrir Square.

In an email, his sister said: "The response from the State Department has largely been one of 'We'll let you know when we know more', but the only contact has been that initiated by my parents."

"They do know that he's being detained at a courthouse, rather than a prison, which we assume is a good thing."

His family voiced their concern for their son's safe return. In a statement issued to the Georgetown Voice, they thanked the "Georgetown University community" for their help, and said: "Seeing the outpouring of love and concern from his friends at a school he cherishes has been valuable moral support for us. We are hopeful that Derrik will very soon be able to thank you himself."

They added: ""At present, we know very little about the situation, though we are in contact with the State Department."

Derrik's father, Kevin, from Jefferson City, Missouri, said his son was a "huge believer in American freedom". He said he had hoped his son's youth would stand him in good stead.

"I'm hoping the fact that he's 19 years old will be a mitigating factor, and hopefully they put him on a plane home," Kevin Sweeney said.

"I suspect what happened is … their government probably isn't absolutely free," he told the Washington Post. "I suspect it's a military government … and I suspect that being with a bunch of Egyptian students, he probably got caught up in something. Who knows?"

"I suppose thoughts of the American revolution were things that probably crossed his mind. He's a huge follower of US constitutional history and the Revolutionary war."

Asked about the molotov cocktail accusation, he said: "I have no idea."

Derrick, who is studying Arabic in the hope it would help a career in law and the military, worked on Barack Obama's election campaign in 2008, but switched allegiances and interned this year with Republican congressman Blaine Luetkemeyer in Washington.

Luetkemeyer's spokesman, Paul Sloca, said he had also been in contact with the State Department and was trying to find out more.

"Our primary concern is that he is safe and being treated fairly," Sloca said.

Sloco said Sweeney came highly recommended and was "very outgoing: a good intern."

The US embassy in Cairo is investigating the detention of Sweeney and two other students at the American University of Cairo: Luke Gates, 21, of Bloomington, Indiana, and Greg Porter, 19, of Glenside, Pennyslyvania.

The three were arrested during clashes outside the interior ministry on Monday, authorities said.

Adel Saeed, a spokesman for the Egyptian general prosecutor's office, told CNN: "The three boys were throwing molotov cocktails, and had no passports on them when they were picked up. They have been questioned by the police and will be further investigated by the Cairo prosecutor."

Egyptian state television showed footage of them standing against a wall, with pictures of their driving licences and ID cards spread out next to what it said were petrol bombs.

Further footage, allegedly taken in Tahrir Square, showed demonstrators – at least one of whom was wearing a mask – with caucasian features, including a young man with blond hair.

A spokesman for the State Department said that no-one from the US embassy had yet met with the students. He said: "Our embassy has been in contact with the Egyptian authorities throughout the day. We have requested consular access and we expect to have it tomorrow."

When asked if they had been charged he replied: "To my knowledge they have only been detained."

A Twitter account that appears to be from Gates – @lhgates – speaks of his experiences in Cairo and of "wanting to die" in Egypt.

On Monday, Gates wrote: "I think I am missing part of my ear" and, a day earlier, told of injuries to his knee and elbow. He wrote: "Back to Tahrir tonight as police set fires to everything, no doubt they will blame it on protesters."

Three days ago he wrote of "feeling reckless" and "honestly, hopefully I die here". Another read: "I just don't want to feel anymore" and one which said: "saw them hanging from the bridge, realise death is the only thing that's immortal."

On what appears to be his Facebook account, Gates changed the current city of residence to Cairo on 31 August this year.

A spokesman for the University of Indiana, where Luke Gates was double majoring in political science and near eastern languages and cultures, said that they had been in contact with his parents. The spokesman, Ryan Piurek, said: "The American University of Cairo and the US embassy are reaching out to him to provide him with any legal assistance he needs."

At Drexel University, where Porter is a student, a spokesman said: "Drexel administrators are in contact with Porter's parents and are working with authorities at the American University in Cairo and the US embassy to have Porter released and returned home safely."

Egypt's generals have claimed foreign intervention is behind some of the violence in Cairo. Foreign media has experienced difficulties attempting to work in Egypt.

Since the Egyptian revolution began at the start of the year, thousands of foreign activists have flocked to Cairo in solidarity with the aims of the Arab spring.

Occupy Wall Street and other Occupy movements, including in the UK, have issued statements of solidarity with Tahrir Square protesters. At a general assembly, Occupy Wall Street voted to send 20 election observers to Egypt at a cost of $29,000 (£18,000).

The arrest of the three students was announced as Egyptians began flowing to Tahrir Square for a fourth day of protests, despite a crackdown by police in which at least 29 people have been killed.

Activists hope to increase the number of protesters in the square, which was the centre of the revolt that ousted Hosni Mubarak in mid-February.

The violence continued, with security forces – backed by military troops – firing volleys of teargas and rubber bullets to block protesters, who responded by hurling stones and firebombs. The two sides have been engaged in intense clashes since the unrest began on Saturday.

State TV reported that three people were killed in the Suez canal city of Ismailia, east of Cairo, overnight.

Hundreds of protesters arrived early on Tuesday to join several thousand who have been camping on Tahrir Square. The crowds hoisted a giant Egyptian flag and chanted slogans demanding that the generals immediately step down in favour of a civilian presidential council.

One man held a sign reading "Ministry of Thuggery" with photos of Mubarak, Hussein Tantawi, the senior military ruler, the prime minister, Essam Sharaf and others.

A few hundred young men nearby chanted "Say it, don't fear, the council must go" and "The people want to execute the field marshal."

On Monday, the civilian cabinet of Sharaf submitted its resignation to the military council, a move that had been widely expected given the government's perceived inefficiency and its almost complete subordination to the generals.