Beverly Coon, the former school board member accused of drugging her boyfriend and setting his apartment afire, will take the stand today as her defense tries to deflect suspicion away from her and, possibly, onto another.

"I've kept this secret for a year," she said, choking back tears shortly after her defense lawyers announced she will take the stand.

What secret Ms. Coon has kept could become clear sometime this morning in the courtroom of Common Pleas Judge Jeffrey Manning.

"She's going to tell the truth as to what happened," said defense lawyer Robert Leight. Ms. Coon's other lawyer, Eric Fischer, said his client asked to take the stand.

Until her arrest last year on charges of stalking, arson and attempted homicide, Ms. Coon was a member of the Baldwin-Whitehall School Board. One month after her arrest, she lost her bid for re-election.

Among details likely to come out today, according to sources close to Ms. Coon, is the emergence of as many as three other anonymous letters. The letters are similar, they say, to one the victim, Ronald Grimm, received in the days leading up to the night of Sept. 8, when police say Dr. Grimm, then superintendent of the Bethel Park School District, was drugged and his apartment set ablaze.

Police accused Ms. Coon of spiking ladylocks, a pastry, with temazepam, a sleep-inducing narcotic, then setting his bed on fire as he slept in a living room chair.

Yesterday, in a sometimes contentious day of testimony during which defense lawyers clashed openly with Judge Manning on procedural rulings, an expert hired by Ms. Coon called the police fire investigation defective.

Carl Natale, a private fire and explosion investigator based in Fort Myers, Fla., said county investigators disregarded basic procedural guidelines, kept inadequate records and failed to consider the possibility that a battery in Dr. Grimm's Apple laptop computer could have exploded, triggering the fire.

The computer was the same model for which 1.8 million batteries were recalled because of fire hazards, although he said the fire appeared to have destroyed the serial number, making it hard to determine if Dr. Grimm's computer contained one of the defective batteries.

Among the faults Mr. Natale found in the county investigation was a failure to measure the depth of charring in wood to determine the length and possible origin of the fire, neglecting to mark the room off in a grid and sift through debris in search of evidence, and laboratory analysis of various fuel sources in the room, including the remnants of the bed on which prosecutors say the blaze began.

"I did not hear anywhere in the testimony that there was any kind of scientific test on any piece of evidence in this case," Mr. Natale said.

Earlier in the day, Lewis Ferguson, an Allegheny County detective who led the criminal investigation of Ms. Coon, said she was found outside Dr. Grimm's apartment the morning of the fire and gave multiple false answers to questions about why she was there and where she lived.

Testimony resumes today at 10 a.m.