Armed police officers have searched a hospital close to the scene of one of four attempted explosions.
Staff at University College Hospital were told in an e-mail to look for a man with wires protruding from his top.
Police were twice deployed to the hospital near Warren Street Tube station, where a man was reported to have run away after the explosion.
Three rooms in an unoccupied part of the hospital remain cordoned off. Two people held near UCH were released.
BBC home affairs correspondent Margaret Gilmore confirmed two people temporarily detained at gunpoint near the hospital were later believed to be unconnected with Thursday's incidents and were released.
Police said there had been no arrests in connection with Thursday's four attempted explosions at three Tubes and on a bus.
The hospital incident was said to be an "entirely unconnected event" from the attacks, by Metropolitan Police head Sir Ian Blair.
Earlier, a man was said to be seen running through the hospital, according to BBC News correspondent Jon Brain.
Police sealed off the hospital
He said a hospital visitor had told him a man had been "pinned" against the wall and kept there for two hours. A police jeep was later seen leaving the hospital.
Reporters had been handed an e-mail saying police had asked staff to be on the lookout for a man on their behalf.
It asked them to look out for a black, possibly Asian man, around 6ft 2ins tall, with wires protruding from a hole in his blue top.
BBC News correspondent Peter Hunt added: "It was an e-mail sent by the hospital authorities. A copy of it was read to me."
At the time, the BBC's Daniel Sandford said: "That adds fuel to the speculation that someone from the Warren Street incident may have escaped to the hospital."
But Professor Jim Ryan, from the hospital, said he had not seen such a memo and dismissed the idea as "absolute rumour".
He said: "We saw armed police going through the hospital. They had a very quick patrol round. We are carrying on working as normal."
Passengers evacuated from Warren Street station had reported seeing a man fleeing the scene.
For two hours staff and visitors were not allowed to leave the hospital while police were called twice to the scene.
Fidelma Fitzgerald was waiting for an appointment in the hospital when the police arrived at 1430 BST.
She saw about 30 and was kept in a treatment room for two hours.
"I saw police running around with guns. That was very worrying," she said.
On Thursday night the hospital issued a statement saying three rooms in an unoccupied part of the hospital remained cordoned off following the search by armed police.
It said police inquiries were continuing.
Routine appointments and surgery on Friday have been cancelled for patients not already in hospital. Staff are being asked to report as normal.
"We are asking patients not to come to the hospital unless they are emergencies," the statement said.
Robert Naylor, the Trust chief executive, said: "Our first priority has been the safety and welfare of patients and staff.
"Throughout this afternoon we have worked closely with the police in their investigations while maintaining emergency services. "We hope to return the hospital to normality as soon as possible."
The hospital treated many of the victims of the 7 July attacks.