They were eager to charge Bomar with the slaying of the 22-year-old college athlete, a police officer's daughter killed on June 20, 1996, after being abducted from an off-ramp on the Blue Route. The detectives had what they felt was good evidence, both circumstantial and scientific. Early DNA tests had pointed toward Bomar.
But the prosecutors wanted more.
Over the next six weeks, investigators would use methods old and new to get the final elements needed to charge Bomar with killing Willard, a crime he denies any part of. They saw a moment of weakness in their suspect and tried to coax a confession.
At the same time, 300 miles away, scientists at a state police lab worked through each painstaking step of the DNA fingerprinting process, seeking conclusive evidence that the DNA found in semen taken from Willard's body was Bomar's.
The prosecutors in the Delaware County District Attorney's Office were in no hurry to bring a charge, sources said.
Bomar, 38, wasn't going anywhere. He was off the streets. Why not be patient and wait for the final DNA evidence to come back from the lab?
That gentle tug-of-war between prosecutors and detectives had been going on for a while when, on Oct. 14, prison officials told county authorities that Bomar had been found with a sheet around his neck. They called it a suicide attempt, though it was unclear whether he had intended to kill himself.
``We thought he might be in a psychologically weak moment,'' a law-enforcement source said.
So, the next day, they hauled him in for questioning. Sources said Bomar seemed ready to say something. At one point, he put his hand over his face and banged the table. But he stopped short.
``He became more interested in listening than talking,'' said an investigator who described how Bomar probed detectives on how much they knew about his alleged connection to Willard.
The connection, detectives felt, was substantial. Tracks near the site where some of Willard's clothing was found matched a tire on Bomar's car. A woman who said she was Bomar's fiancee told investigators that he had planned to go to Smokey Joe's in Wayne on June 19, the day Willard went there with friends.
And, perhaps most incriminating, marks on Willard's body were determined to match the oil pan on the underside of a green, 1993 Ford Escort that Bomar owned.
But having failed to extract a confession, investigators sent Bomar back to his cell in Camp Hill, where he remained until last week, when prosecutors revealed what they are confident is the final link: DNA evidence indicating that Arthur Bomar raped and murdered Aimee Willard.
Delaware County District Attorney Patrick Meehan said those results arrived within the last few weeks. They were analyzed over and over, and on Wednesday - nearly 18 months after Willard's body was found in a trash-laden lot in North Philadelphia - Bomar was formally charged with her killing.
At his arraignment last week, he told reporters, ``It's not my DNA.''
* However sure investigators may be, DNA evidence remains controversial.
As defense lawyer Barry Scheck emphasized in the O.J. Simpson trial, human error can lead to contamination in the lab and to false results.
``Techniques have evolved and are constantly being upgraded and improved,'' said Lawrence Kobilinsky, a professor of forensic science at the John Jay College of Criminal Justice in New York and a frequent expert witness in court on DNA matters.
``But it depends on the type of testing done. We have still not reached the point where DNA sequencing, the most reliable process, is done routinely in the lab.''
But Bruce Weir, a professor of genetics and statistics at North Carolina State University, calls DNA ``extremely powerful evidence.''
``With the current technology, you can get so much DNA from a sample that the evidence can become overwhelming,'' said Weir, who has appeared as a prosecution witness in trials ranging from Simpson's to that of a suspected rapist in Media.
The job of putting the scientific case together fell to a team of scientists in goggles and gloves working in the Pennsylvania State Police DNA laboratory in Greensburg.
Using powerful microscopes, chemical solutions, tiny test tubes, measuring devices and computers, Sara Gottwald and Christine Tomsey were able to isolate what they say is the DNA, or unique genetic code, of Willard's assailants from semen obtained from a vaginal swab.
Those samples were compared to DNA extracted from a sample of Bomar's blood taken by investigators after he was arrested on burglary charges on June 5.
DNA tests can take anywhere from a few weeks to six months, because mapping the sequence of the four chemicals that make up the genetic blueprint of each person is a painstaking process, Tomsey said. Dozens of tests are required to map the unique DNA structures.
The tests, though, can also exclude a suspect. That's what happened in the case of Andrew Kobak, the Bryn Mawr man initially questioned by police. His DNA did not match the sample from the sperm, and he was exonerated.
If similarities persist, though, the tests become more painstaking and time-consuming, Tomsey said. Throughout the process, DNA results from Bomar's blood kept coming back with similarities.
Lab notes are analyzed in a special, FBI-designed computer program. The similarities are reviewed by other technicians and Tomsey before police investigators receive a final report - which can be as long as 50 pages, Tomsey said.
According to Tomsey, it is possible to construct a match that narrows the field to ``one person in 1,000'' or ``one person in a million or billion.''
Neither she nor the prosecutors, however, would say how precisely Bomar's blood matched the semen found in Willard's body. The affidavit simply read: ``The two profiles match.''
Willard's parents were told on Tuesday evening that there would be an arrest the next day. For Gail Willard, it marked the start of ``another part of the long and painful journey,'' as she learns more details of her daughter's murder.
``Our only comfort,'' she said, ``is the knowledge that Aimee is safe now and evil will never hurt her again.''