In his 17 years as a defense lawyer, Edward Smith Jr. has seen the hardest of hardened criminals. Peter and Vicki Snyder, the young Navy couple charged with killing their daughter, didn't fit that profile.

"From my first meeting with them at the time of arraignment last September, I thought something was wrong," he recalled. "They just didn't strike me as the kind of people who would consciously withhold something from their child."

In fact, the entire defense team began to sense early on that this couple from a tiny hamlet in northwestern Pennsylvania could not have been so abusive as to deny a 9-month-old child food and fluids for nearly two weeks, as the prosecution claimed.

"Red flags kept going up," said Tyler Johnston, the lead lawyer on a defense team that persuaded a federal judge on April 11 to take the unusual step of acquitting the couple before the case went to the jury.

There were neighbors angered by the charges, hospital records that showed the couple had repeatedly sought medical care for their daughter, and a nationally recognized expert in pathology who disputed the findings of a state medical examiner.

And just a week before the trial started, defense lawyers made a startling discovery. They learned that a blood culture critical to the autopsy findings had never been completed by state microbiology laboratories.

Dr. Joseph Libonati, head of the lab, would not comment. Dr. Frank Peretti, the assistant medical examiner who performed the autopsy, referred questions to Dr. John E. Smialek, the chief medical examiner.

Dr. Smialek said Friday he is "undertaking my own, internal review into what happened. And I'm asking other outside experts to look at it."

Gregory Welsh, the assistant U.S. attorney who prosecuted the case, would not comment. He said the Naval Investigative Service agents who gathered evidence against the Snyders have since been transferred to overseas posts.

The unraveling of the charges started with neighbors at the Patuxent River Naval Air Station in St. Mary's County, where Mr. Snyder is stationed.

"They told us, unsolicited, about crib bumpers and gates across the stairways and baby monitors," said Mr. Johnston. "That was the first red flag. Why would they have all those things if they were just going to ignore their child?"

Add to that the voluminous medical records. From the time their daughter, Alyson, was born in February 1989 until she died the day before Thanksgiving, the Snyders made nearly 20 trips to doctors, including three emergency room visits, because of the baby's poor health.

"If you're neglecting a child, you don't want anyone to find out, you don't keep taking her to the hospital, emergency room and doctor's office," said Mr. Johnston, an assistant federal public defender.

But prosecutors had a medical examiner's report that insisted Alyson, who was found dead in her crib, was severely dehydrated and had an acute electrolyte imbalance. It was a condition, the autopsy concluded, that could have been caused only by a lack of fluids.

Prosecutors also had reports from Naval Investigative Service agents who searched the Snyders' home on the base about 1 a.m. two days after Alyson's death and found a mess in the baby's room, with clothes, toys and blankets tossed about.

"What was weaved through the whole story was these people were untidy housekeepers, therefore they must have killed their child," complained Joseph Segretti, a former Baltimore homicide detective who now works as an investigator in the federal public defender's office. "That wasn't the case."

But the problem was to build a case for the Snyders, who have two other children, Amanda, now 3, and Zachary, who was born two weeks after the arraignment. The defense team started with what the prosecution didn't have -- a motive.

"We could never find someone who heard them say I hate this kid, I wish I never had her," Mr. Segretti said. "All the witnesses said they were untidy, but nobody ever said they would abuse their kids."

In fact, the neighbors were outraged.

"It was absurd," said Sharon Lyons, a former neighbor. "I've seen the way they treated their children, and I couldn't believe they were talking about Pete and Vicki. I'd trust them with my children in a heartbeat."

The defense had plenty of friends willing to testify that the Snyders, both now 21, were good parents and that they saw Alyson being fed repeatedly in the days before her death. But they didn't have another explanation for Alyson's death.

Mr. Smith originally theorized that she had a violent, allergic reaction to her formula. But he dropped that, he said, after several pediatricians told him that that was all but impossible.

Dr. Michael Frac, Alyson's pediatrician, and Dr. Peretti had suspected meningitis, but their tests were negative. Yet other medical sources told the defense team that that didn't necessarily rule out the disease.

It is not at all rare for the organisms that cause a blood-borne infection not to show up in such tests, said Dr. Cyril Wecht, author of a three-volume text on forensic pathology, who reviewed Alyson's medical records and the autopsy report. It was the photos in that report that prompted his conclusion, he said.

"The skin of the body showed clear evidence of mottling. That dusky, intense red, purplish discoloration of both feet and splotches on the thighs, the smaller pinpoint hemorrhages are all classical for meningococcemia, a blood-borne infection," Dr. Wecht said, concluding that Alyson died from the shock of infection with the beginnings of meningitis.

Angry at what he called a "reprehensible" conclusion that stemmed from "a mind-set that if the house was dirty, they abused their child," Dr. Wecht testified for the defense.

With two weeks to go before the trial, the defense team learned that the blood culture that should have been done within days after Alyson's death had never been done. And when it was completed, the results were inconclusive.

"I wasn't sure what it meant, but I thought I was looking at reasonable doubt," Mr. Johnston said. He went looking for an expert on infectious diseases and found Dr. Patricia Charache, head of the microbiology laboratories at Johns Hopkins Hospital, late on Good Friday afternoon, a week before jury selection.

As he explained test results and symptoms over the telephone, she said it pointed toward meningitis and asked if he could describe the baby's body. Better than that, he said, he had pictures.

"Without ever having seen the pictures, she was describing this baby exactly over the phone," Mr. Johnston recounted. "I had chills going up and down my spine."

Within minutes, Mr. Johnston had loaded his files and Mr. Segretti into a car, and they were driving through the rain and holiday traffic to Hopkins Hospital. For three hours, Dr. Charache reviewed and copied reports and called other doctors for consultation.

"He had been so traumatized by other experts who told him it couldn't be meningitis that I wanted him to hear it from someone else besides me," Dr. Charache said.

"She had an explanation that fit everything else we couldn't explain," Mr. Johnston said.

When the trial began, the defense team was confident but apprehensive, Mr. Johnston recalled.

"I knew we had innocent clients, and the only way they could be convicted was if we did a bad job," he explained. "We knew we were standing between innocent people and jail. That scared the hell out of me."

The Snyders were prepared for the possibility they would lose.

"I was thinking about all the birthdays and Christmases I would miss," Mrs. Snyder recalled. "It would be Zach's first real Christmas."

"It was the ruining of our future," Mr. Snyder thought.

They knew that they were facing 10 years in prison and that they could have got off with less than a year if they had they pleaded guilty to the lesser charge of involuntary manslaughter, but "they wouldn't do it," Mr. Johnston said.

"We weren't going to admit to something we didn't do," Mr. Snyder insisted.

At the trial, Dr. Wecht and Dr. Charache sharply contradicted the autopsy report. Mr. Snyder, who testified for nearly an hour, was riveting as he recounted the day he and his wife found Alyson dead in her crib.

At last, when all the witnesses were finished, Tony Gallagher, an assistant federal public defender brought in to help about a week before the trial, asked Judge Herbert F. Murray to acquit the couple because there wasn't "sufficient evidence to indicate that the conduct of the Snyders was in any way abusive."

To nearly everyone's surprise, including Mr. Johnston's -- he was back in his office preparing his closing argument -- Judge Murray granted the motion. It was over.

Later, Mr. Johnston would fulminate about a "witch hunt" conducted by the Naval Investigative Service and complain that the Snyders, a poor Navy enlisted couple, were the victims of "class prejudice."

But Mr. Smith was more philosophical. "The government had its point of view, and they had to go forward," he said. "But justice was done in the end. That's what our system affords us."

More importantly, he added, "The spirit of that child has finally been released. That child is now free to rest, and her parents can go on with their lives."