Mr Elendu runs a controversial Nigerian news blog
A US-based Nigerian news blogger is being held without charge by Nigeria's secret service.
Jonathan Elendu was taken into custody on Saturday when he arrived in the capital, Abuja, on a family visit.
The State Security Service (SSS) has refused to allow his lawyers access to him and denied him a medical visit.
Elendureports.com is one of a number of diaspora-run "citizen reporting" websites about Nigeria and is known for publishing controversial stories.
According to Nigerian law anyone arrested must be charged in court within 48 hours, but correspondents say the rule is frequently broken.
The SSS told Mr Elendu's lawyer that Mr Elendu had not been "arrested", but "invited" for talks at their headquarters.
An SSS spokesman said he was being investigated for "acts of sedition", but refused to give details.
Spokesman Kene Chukwu also told the BBC that Mr Elendu's detention had followed legal rules.
"I am telling you all the legal rules were followed, and you have to accept it," Mr Chukwu said.
Mr Elendu's lawyer says he has not spoken to his client since his arrest.
"They have not pressed any charges and have not allowed anyone to see him," said Ugo Muoma.
He said he was filing papers in court to force the SSS to charge or release Mr Elendu.
Elendureports.com operates from Lansing in Michigan and publishes often controversial stories about Nigerian politicians, accusing some of them of corruption and other crimes.
Their stories are often based on anonymous sources.
Another US-based Nigerian news website, Saharareporters.com, quotes anonymous sources as saying Mr Elendu may have been arrested because of photographs it published a few months ago showing President Umaru Yar'Adua's son.
Elendureports.com once mistakenly reported that Mr Yar'Adua had died
The Saharareporters.com pictures, which caused a stir in the local media at the time, showed 13-year-old Musa Yar'Adua waving wads of money around and holding a policeman's gun.
But Saharareporters.com says Mr Elendu is not a member of their staff and has nothing to do with the photographs.
International media rights groups Reporters Without Borders has called for Mr Elendu's release.
"There haven't been many really controversial stories about the president on Elendureports.com in the last few months," said Mr Muoma.
During the election campaign in 2007, Elendureports.com claimed that Mr Yar'Adua had died during a medical trip to Germany.
Two foreign journalists have been detained and deported by the SSS for reporting in the politically sensitive oil-rich Niger Delta region over the last few months.
In September, six local reporters and media executives were detained and questioned after a television channel reported, after receiving a hoax e-mail, that the president planned to resign.