"It's intimidation," he said. "And what is more intimidating to someone than knowing their family could be firebombed? "

"The public should be outraged," said Police Commissioner Sylvester M. Johnson. "Six innocent people killed, burned to death, in their own home. "

Death by child abuse

In the hours before he died, 3-year-old Luis Rivera Jr. would have been in agony - crying and vomiting, the city's assistant medical examiner said. His liver was crushed, his pancreas damaged by blows to his tiny body.

His mother, Desiree Pizzaro, is being held without bail on murder charges along with her boyfriend, Victor Santana. Both have pleaded not guilty.

Luis' death in August is the fifth homicide this year that police classified as child abuse.

The others, police say, were a 7-year-old beaten by parents, a 5-year-old hung by the mother's boyfriend's brother, and a 3-year-old and a 6-month-old each killed by their fathers.

Their deaths are a harsh reminder that children, especially infants, are sometimes killed by those closest to them.

Since January 2000, police have investigated the deaths of 39 children killed because of child abuse.

But those deaths may be only a part of the picture. The city's Department of Human Services has a higher tally: 55 child abuse deaths, including 11 this year. DHS counts some deaths that don't fit the legal description of homicide.

When investigating child-abuse cases, police often find that someone knew that the child was being beaten and stayed silent.

"There are usually people in families who know something is wrong," said Inspector James M. Boyle, head of the Police Department's Homicide Division.

Last year, 1,200 children were physically abused in Philadelphia. Such children, studies show, are at risk of becoming abusive adults.

For this reason, DHS and the city health department are aiming parenting programs at the most violent neighborhoods.

"We are trying to keep infants and children from being hurt," said Katherine Maus, who directs the department's division on maternal, child and family health, so that when they grow up "they are less likely to go out and hurt other people. "