Lawyer Peter M. Villari told The New York Times that officials of the Camden County prosecutor's office went over details of an autopsy report with him, the newspaper reported Tuesday.
The boys, ages 5, 6 and 11, disappeared from the yard where they were playing a little after 5 p.m. on June 22. Their deaths were ruled accidental.
Relatives searched for the boys for three hours and then called authorities. A two-day search that included dogs, helicopters and boats on the nearby Delaware River ended when the father of one of the boys found them dead in the trunk of an inoperable car sitting just feet from where they had been playing.
The autopsy report, which has not been released publicly, found that the boys died between 10:30 a.m. on June 23 and 2:30 a.m. on June 24, Villari said. That estimate was based on fluid and tissue samples and weather data, he said.
Villari told the newspaper that responsibility for the deaths is now "squarely on the shoulders of the police."
"I think the numbers speak for themselves," he said. "They were certainly alive when the police arrived and certainly well after the search started."
Villari did no immediately return a call seeking additional comment Tuesday. The prosecutors' office had no immediate comment.
Several experts told The Associated Press last month that it was likely the boys would have passed out within an hour or two of becoming trapped in the trunk.
The questions of when Anibal Cruz, Jesstin Pagan and Daniel Agosto died is crucial in the case, in part because of the possibility of lawsuits against officials.
So far, no legal complaints have been filed.
Villari said last month that he was hired by Anibal's mother, Elba Cruz, in part to see if police or anyone else may have been responsible for the boys' death. He said at the time that it was not clear whether she might sue.
A report from prosecutors on issues including why searchers never looked in the car trunk is due next week.