The authorities are focusing their efforts on several individuals — including a laboratory technician — known to have been in the basement of 10 Amistad St. at the time when Annie Le GRD ’13 was murdered, according to three sources familiar with the investigation.

A spokesman for the New Haven Police Department, Joe Avery, said at 1 p.m. Monday that there were no suspects in the investigation. But by 10 p.m., Avery no longer denied that authorities have narrowed in on a few people of interest.

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“We have nothing to give out at this point,” he said.

Nevertheless, the Office of the Chief Medical Examiner in Connecticut confirmed yesterday that the body found in the Yale research facility on Sunday was Le’s. The medical examiner did not release the cause of death; officials at the office said they would announce it this afternoon.

The medical examiner did rule the death a homicide, though Avery said there is no reason for students to fear for their safety.

“We are not looking at it as if it is a random act,” he said.

At a meeting of the medical school community Monday afternoon, several audience members — concerned by evidence that the killer had a Yale ID and access to the basement of 10 Amistad St. — asked if the perpetrator is still at large. University President Richard Levin responded by saying that he has confidence Le’s homicide will be resolved. Because security systems in the building recorded who entered the basement and the times at which they entered, the number of potential suspects has been limited to a very small pool, Levin explained.

The appropriate people are being monitored, and there is no reason to worry about working in Yale research facilities, Levin said.

“The people in the basement aren’t going to cause any trouble until the matter is resolved,” he said.

A University official familiar with the investigation confirmed media reports that polygraph tests have been administered to some of the people known to have been in the basement last Tuesday. Still, the results of those tests have not necessarily produced reliable information that could advance the investigation, the official said.

Law enforcement officials continue to focus their efforts on the people who had access to the basement and who may have encountered Le after she entered the building at 10 a.m. last Tuesday, Levin said. Access to the basement area where her corpse was found is a restricted to certain specially authorized individuals who must use their Yale identification cards to access the floor.

With Le’s killer presumably still at large, Vice President and Secretary Linda Lorimer announced Monday that the University would increase security in the area near Amistad Street immediately.

She said Yale has increased patrols in the area and added security officers inside the Sterling Hall of Medicine, where Le, who was supposed to be married Sunday, had an office. Le left her purse, wallet and phone in this office before walking to 10 Amistad St. last Tuesday.

The research facility where Le was killed was closed Monday — and will remain closed today — except for those with “essential research responsibilities.” When the building reopens entirely, Lorimer said, extra security officers will be stationed inside and outside “for the foreseeable future.”

It is not clear what access — if any — there currently is to the area where Le’s corpse was found. Avery, the police spokesman, said the remains were discovered in a chase in the basement, a kind of shaft that provides room for mechanical equipment to travel from the roof to the basement of the 120,000-square-foot building.

The authorities found Le behind that basement wall after searching the building for four days. They could not find any video footage of Le leaving the building, and so the search until Sunday was focused inside the building. Dogs from the Connecticut State Police were even brought in to help investigators.

In his address to the medical school community Monday afternoon, Levin explained that the material of the wall behind which Le’s body was hidden made it difficult for dogs or humans to locate her.

“It was only after a number of days that the scent became detectable,” he said.

Avery said that state police, as well as the Yale Police Department and the Federal Bureau of Investigation, will remain involved in the case for “as long as they are needed.” Because the case is now a homicide within city limits, the NHPD has jurisdiction over the matter.

Isaac Arnsdorf, Nora Caplan-Bricker and Colin Ross contributed reporting.