About 10 minutes later, the witness said, he heard a shot and saw Mr. Maldonado running from the building’s door. Then he said the woman and her companions came out of the building. The woman handed a pistol to one of the men, who tucked in his waistband. “She passed the gun away after the shot,” he said.

The police said the other witness also saw the woman hand a gun to one of the men before all three fled toward 156th Street.

Detectives had questions about these accounts. Both tipsters said the shooting took place in the building’s lobby, but the spent shell casing and the unfired round were found outside. And neither person saw who fired the gun. “We don’t know who the triggerman was,” said Detective Jeff Meenagh, the lead investigator.

Detective Meenagh tracked the last call on a cellphone found on Mr. Maldonado’s body to a woman living in the building. She admitted buying $20 worth of crack from Mr. Maldonado in a third-floor hallway, but said she knew nothing about the shooting. She passed a lie-detector test.

Investigators never found a second cellphone that Mr. Maldonado owned and that he dropped as he ran, the witness who spoke to The Times told the police. They learned its number from an associate of Mr. Maldonado’s who was arrested in a drug sweep a few days after the murder. Call records show Mr. Maldonado did not receive a call on that phone just before the shooting.

To make matters more complicated, the detectives identified the two men seen with Mr. Maldonado’s former lover as violent felons, both recently released from prison. One had been robbing drug dealers in the projects, the police believe. Mr. Maldonado had $126 on him when he was killed and a bag of drugs, but a gold chain he had bought two days earlier was missing, his friends said.

Now, investigators say they need more leverage to crack the silence in the Melrose Houses: the arrest of someone who saw the shooting or has detailed, credible knowledge about what happened.