When Richard A. Coffman's 5-year-old brother died in 1972, Coffman was only 3.

Although his recollection of his brother's death was fuzzy at best, his two-paragraph e-mail to Baltimore County police this summer said he knew who did it: his mother.

This week, 32 years after Edward Coffman's death, Baltimore County police went to Orange City, Fla., and arrested Diane B. Coffman, 57, at the dental office where she works as a receptionist.

She is charged with first-degree murder in her son's death.

"It is quite an unusual case," Baltimore County police spokesman Bill Toohey said yesterday. "It's old, the mother is accused of killing her child, and the tip came from another child."

Toohey would not say specifically what was in the e-mail sent by Richard Coffman, 35, other than that it was about his mother's character and things that had happened after his brother's death. Richard Coffman could not be located for comment yesterday.

"It wasn't so much what he remembered about age 3, but some other things about his past that he shared with us," Toohey said.

Diane Coffman has been in jail in Volusia County, Fla., since her arrest Tuesday. No one answered the phone yesterday at the home she shares with her husband, Darryl Coffman, in DeLand, Fla.

The case was reopened in July after Richard Coffman's e-mail grabbed the attention of two Baltimore County detectives, James Tincher and Philip Marll.

At the time of Edward Coffman's death, Diane Coffman told police that her son had fallen in a bathtub at 4 p.m. Aug. 3, 1972, in their Baltimore County home near Woodlawn, according to police.

She said she put her son to sleep at 8 p.m. and then found him unconscious in his crib at 4:30 the next morning.

An autopsy showed Edward Coffman had 17 injuries on his body, some that were fresh and others that were healing.

The cause of death was ruled by the medical examiner as trauma. But the manner of death, homicide or accident, was left as "undetermined."

It wasn't until the case was reopened in July that authorities figured out that the injuries were inconsistent with the story the mother told in 1972, according to police. "The injuries were sustained over a period of time," Toohey said, "and that indicated some kind of abuse."

Reviewing that information, Maryland's chief medical examiner, David R. Fowler, changed the manner of death last month from undetermined to homicide.

Detectives tracked Coffman and her husband to DeLand, on Florida's east coast. The couple had moved there about three years ago, and she was working at Shah Dental Services in nearby Orange City.

The Maryland detectives showed up without warning at the dental office about 8:40 a.m. Tuesday. They had with them two deputies from the Volusia County Sheriff's Office. Coffman was led away without incident, said Brandon Haught, a spokesman for the sheriff.

"They just knocked on the door," Toohey said. "That's what they usually do. The usual pattern is to just show up."

Baltimore County police were expecting her arrival in Maryland as early as today.

At the dentist office, Coffman was known as a reliable employee and a kind woman who tacked up pictures of her 3-year-old grandson at her desk, colleagues said. They said Coffman told them that the boy was her daughter's child.

"She never mentioned she had any sons," said Lata Shah, who is the office manager and whose husband owns the dental practice. "She has talked about her daughter. I thought she had only one child."

Shah said the office staff was stunned to see one of its employees led away by police.

"It was a kind of shock to me," Shah said. "None of us knew anything about this. My husband asked the police if he could help them, but they told him he shouldn't ask anything."

Property records show that Diane and Darryl Coffman have lived in several places in Maryland, including Gwynn Oak, Randallstown and Hampstead. In 1997, they declared bankruptcy while living in Sykesville.

Staff researcher Bobbye Pratt contributed to this report.

Diane Coffman was arrested at work in Florida.