The women of the family left Fernard B. Strowbridge in charge of 10 children one day in October. While they were gone, a D.C. Superior Court jury concluded, Strowbridge battered his 8-month-old nephew, who died four days later.

The jury's verdict of first-degree felony murder this week makes Strowbridge the first D.C. defendant to face stiffened penalties for fatal child abuse. He must be sentenced to at least 30 years in prison.

Prosecutors and children's advocates pressed for the 1997 law out of frustration. They contended that killers were serving less time than they deserved because juries would rarely convict a person of first-degree, premeditated murder when a child was involved.

A Washington Post series last year detailed the difficulties of accurately identifying a child abuse death and bringing a successful criminal case, particularly in infant killings. Shortcomings in initial medical and police investigations of the deaths cause many to be wrongly labeled as deaths from illness or accident. Even in cases where an adult is held responsible, the series showed, punishment can be minimal because relatives refuse to testify and juries refuse to believe that an adult intended to kill a child despite evidence of brutal beatings, shakings and other abuse.

A provision that took effect last year in the District allows juries to conclude that someone guilty of first-degree cruelty to children is also guilty of felony murder if the child dies. The murder conviction, in turn, triggers a sentence of 30 years to life.

Prosecutors in the District have filed charges in three child homicide cases this year, along with three last year. In 1998, prosecutors filed charges in 72 misdemeanor child abuse cases and about three dozen felony cases, said Assistant U.S. Attorney Julieanne Himelstein.

D.C. jurors heard from witnesses that Strowbridge, 32, was baby-sitting for 10 children on the afternoon of Oct. 18 while the mothers were out of the house. Among the children was his infant nephew, Dhani Brown, who had a cold and was feeling cranky.

Two of the older children wanted to comfort Dhani, but Strowbridge wouldn't let them, Assistant U.S. Attorney Ryan Rainey told the jury. At one point, Strowbridge took Dhani from the room, then rushed back in to say the boy had suffered a seizure.

Strowbridge "had had enough, and he was going to stop the kid from crying," Rainey said.

An autopsy showed that Dhani, who died Oct. 22 at Children's Hospital, had 17 bruises on his head. D.C. Medical Examiner Jonathan L. Arden testified that the injuries were consistent with someone hitting or kicking Dhani or "swinging or shaking or propelling" the child into a hard object.

The jury convicted Strowbridge, formerly of Northeast Washington, of first-degree cruelty to children, first-degree felony murder and second-degree murder. Judge Stephen G. Milliken will sentence him July 7.