CAIRO (AP) — Lawyers for a leading Egyptian investigative journalist and human rights advocate said Monday that the military is holding him in an undisclosed location while he faces charges of spreading "false news."

Adel Ramadan, one of the lawyers, said military prosecutors would not tell him the whereabouts of Hossam Bahgat, who was detained Sunday after being summoned to an intelligence building in Cairo.

Bahgat is one of Egypt's best-known rights advocates. He founded the Egyptian Initiative for Personal Rights in 2002, was honored with a Human Rights Watch Alison Des Forges Award in 2011, and just returned to Egypt after spending a year as a visiting fellow at Columbia University's journalism school in New York.

The detention, which caused a wave of uproar on social media and among activists, is part of a sweeping crackdown on critical media launched in the aftermath of the military overthrow of Islamist President Mohammed Morsi in 2013.

Ramadan said prosecutors asked Bahgat in detail about an article he wrote last month which described the August 2015 conviction of a group of military officers on charges of conspiring with Morsi's banned Muslim Brotherhood to plot a coup against President Abdel-Fattah el-Sissi, who as military chief had overthrown Morsi.

"I asked the head of the military prosecution the place of detention, and was told I would not be told," Ramadan said, adding that prosecutors have ordered Bahgat held until at least Wednesday, when his lawyers have been summoned for another meeting.

"Nobody knows what will happen then — maybe he will be released, maybe they will hold him longer," he said.

Military spokesman Brig. Gen. Mohamed Samir said Bahgat has been referred to military trial for "compromising national security" and writing about the army without written permission from its leadership. If found guilty, Bahgat could serve up to one year in prison and/or pay a fine of up to $2,500, he said.

Since Morsi's overthrow, Bahgat has written a series of highly detailed and meticulously researched investigative pieces, including the article in question. That article was based on official documents, including the military prosecutor's indictment, and interviews with the military officers' families.

"He is being detained and questioned by the military prosecutor for peacefully exercising his right to freedom of expression and must be immediately and unconditionally released," said Philip Luther, Director of the Middle East and North Africa Program at Amnesty International.

"Any charges brought against him must be dropped," he said.

Ramadan said prosecutors presented no evidence against Bahgat. He said they asked Bahgat to promise to stop writing about the military but he refused. Bahgat did, however, offer to publish any necessary corrections or clarifications, his lawyer said.

The crackdown launched after Morsi's overthrow initially focused on his Islamist supporters but was soon broadened to other dissidents, including secular activists who led the 2011 uprising that ousted longtime autocrat Hosni Mubarak.

In June, the New York-based Committee to Protect Journalists said reporters face unprecedented threats in el-Sissi's Egypt. CPJ said Egypt currently had the highest number of journalists behind bars since it began keeping records in 1990. It said the threat of imprisonment in Egypt is part of a stifling atmosphere in which authorities pressure media outlets to censor critical voices and issue gag orders on sensitive topics.

Egyptian officials say no one has been jailed over their journalism, and that those facing criminal charges related to other matters must be tried by judicial authorities without the government's interference.

Bahgat published his articles in English and Arabic in the independent online publication Mada Masr. The chief editor of the publication, Lina Attalah, said press freedoms in Egypt have been severely curbed in recent years.

"We are working as usual," she said. "This is part of the continuing crackdown... We are just calling for his release."

Associated Press writer Nour Youssef contributed to this report.