Investigators searching for a suspect in the killing of Linda Stein could be focusing their case on the people repairing the roof of the famed real estate broker’s Fifth Avenue home, a project that had infuriated Stein in the weeks before she was killed, a source said. During a meeting at her apartment days before she was killed, Stein, 62, shouted at the superintendent of her building, Edmond McQuaid, because of the damage that the roof repairs had caused to her 18th floor apartment, a source close to Stein said. Stein was livid because the roof work had caused cracks in the ceiling and walls of her apartment at 965 Fifth Ave., the source said.

Mr. McQuaid could not be reached for comment.

While roof workers at the building have said they provided investigators with DNA samples, police officials would not comment on details of the investigation.

“All I can tell you is that we’re interviewing family, friends, employees of the building, as well as the construction people working there,” the head spokesman for the police department, Deputy Commissioner Paul Browne, said. “No suspect has been identified.” Wonder Works Construction Corp. is performing the roof repairs on Stein’s building. A phone call and faxed message to the company asking for comment were not returned. The company that manages the building, Brown Harris Stevens, declined to comment. It is unlikely that a suspect could have sneaked into the co-op apartment building, friends and residents said.

“The security in the building is meticulous,” a resident in Stein’s building who has vast experience dealing with security systems in the city said. “People have major art collections. It’s run like a battleship.”

Other than family members, there were two people that had regularly been in and out of Stein’s apartment in the weeks before her death, her assistant, Natavia Lowery, and a colleague at Prudential Douglas Elliman, Marc Benecke, a source close to Stein, said.

Stein and Mr. Benecke had worked together for about 20 years after meeting at the nightclub Studio 54, a real estate consultant, Esther Muller, who worked with Stein at the Real Estate Academy, said.

“I know they had a good working relationship,” she said.

In recent months, Stein had been working from home more often, which is likely why Ms. Lowery was spending extra time at her apartment, a friend and colleague, Faith Hope Consolo, said.

Telephone calls placed to both Mr. Benecke and Ms. Lowery were not returned.