Egyptian police investigated Italian student Giulio Regeni shortly before his abduction, torture and murder, Egyptian and Italian prosecutors say.
But the inquiry was dropped after concluding he posed no threat, the prosecutors add.
It is the first official Egyptian acknowledgement that Regeni was being monitored by the security services, correspondents say.
His body, showing signs of torture, was found dumped near Cairo on 3 February.
The 28-year-old Cambridge University student had been researching trade unions, a politically sensitive subject in Egypt, when he went missing in Cairo on 25 January.
No-one has been arrested over Regeni’s death although in March Egyptian authorities said they had found a criminal gang responsible for his kidnapping and murder.
All the gang members were killed in a shoot-out, they said. The reports have been branded "implausible" by academics who have criticised the Egyptian authorities.
"Police probed [Regeni’s] activities for three days before concluding that his activities were of no concern to national security, and based on that [conclusion] dropped the investigation," a joint statement signed by Egyptian and Italian prosecutors said on Friday.
The statement said Egypt’s prosecution service was still investigating the gang’s links to Regeni’s abduction.
Egyptian and Italian prosecutors are also trying to retrieve footage from CCTV cameras from a metro station Regeni is believed to have entered before being abducted.
The process of retrieving the deleted footage is expensive and Egypt ha asked Italy to help, Egyptian officials said. They also accused Italian prosecutors of "unconstitutionally" demanding thousands of phone records which Egypt has refused to hand over.
Police initially suggested Regeni had been killed in a road accident, and have since offered little information on the progress of their investigation.
An Italian autopsy revealed that his body was covered with cuts and had numerous broken bones, suggesting he had been struck with "fists, batons and hammers".
A letter "X" in addition was carved on his forehead and hand, according to Italian media.
Regeni’s parents Paola and Claudio have joined calls for the Egyptian authorities to co-operate more fully with the Italian government.
In April, the UK Foreign Office condemned "the brutal manner of [Giulio Regeni’s] killing" and admitted it was "disappointed by the limited progress made in the case".
*A previous version of this article stated that Egyptian authorities said they had detained Regeni; they said only that they had investigated him.