Thomas, 44, of Bala Cynwyd, had served time in federal prison for counterfeiting and was on probation in Montgomery County for forging $140 in grocery-store gift certificates. He also had been a suspect in the 1999 unsolved disappearance of his former girlfriend.
One law-enforcement source said Thomas had vowed to friends he would never go back to prison.
Those odds changed dramatically just after 5 p.m. Thursday.
According to the account released by Montgomery County District Attorney Risa Vetri Ferman and Plymouth Police Chief Joseph Lawrence, Thomas was driving a stolen Infiniti SUV when he came upon a traffic jam at the scene of an auto accident on East Ridge Pike.
Thomas, they said, swerved the silver SUV into the opposing lane and sped past the backlog. In the process, he struck a Chrysler sedan.
Already at the scene, Fox jumped into his patrol car and gave chase, investigators say. He found the Infiniti abandoned on Ernest Station Road, along the edge of the river trail.
After radioing for backup, Fox took off down the path. A newly minted K-9 officer, he brought his police dog, Nick, on the chase.
Thomas was hiding in tall weeds on a bluff above the trail when Fox came into view below, the prosecutor's account said.
Both men fired, according to Montgomery County Coroner Walter Hofman, whose office examined the bodies.
One shot struck Fox in the head, killing him instantly. Another grazed his dog, a Belgian Malinois, in its leg.
But Thomas was also hit. Officers found him with two gunshot wounds to the chest.
"One, more likely than not, was the result of the shot from Officer Fox," Hofman said. The second, fired at close range, indicated that Thomas had fatally shot himself, the coroner ruled.
Hofman said the bullets passed through the men's bodies and weren't recovered. But investigators retrieved both guns at the scene.
"Just two people: the officer and the perpetrator," Hofman said. "There's no third party."
Brett Datto, a lawyer who represented Thomas, said Thomas was a Lower Merion High School graduate who was close to his mother. Datto said that Thomas' family was trying to gather more information before speaking publicly, but that he was skeptical Thomas killed himself.
"That does not comport with my representation and knowledge and him over the past 15 years," Datto said.
Ferman said the investigation was continuing, including an examination of ballistics evidence.
Thomas had once been a prime suspect in the 1999 disappearance of his ex-girlfriend, said William Colarulo, who was the Philadelphia police captain overseeing detectives in the case.
The woman, Maria Procopio, 35, of South Philadelphia, was last seen leaving her job at QVC in West Chester.
Throughout the day Friday, a parade of tearful friends, colleagues, and relatives streamed to Fox's two-story home in Gilbertsville. At least one fund had been set up to collect donations for his family.
Funeral arrangements are pending.
At the Plymouth Township municipal complex, officials draped black bunting over a sign and made plans to retire Fox's badge number, 178.
Detective Sgt. Thomas Longo, a friend and colleague, noted that Fox endured two tours in Iraq and once survived an IED attack while riding in a Humvee. But Longo also remembered him as the avid Flyers fan and enthusiastic father who brought his newborn girl to the office when she was barely a week old.
"He loved his wife, he loved his baby, he loved his country, he loved being a police officer," Longo said as flower deliveries and other gifts streamed into the police department. "Going forward is going to be very hard, but we're police officers, that's what we do. We go forward."
Friday night, more than 500 people stood vigil outside Fox's home on what would have been his 35th birthday.
The killing of a police officer hits the suburbs extra hard, said an officer from Tredyffrin Township, who declined to give his name.
Asked how many such funerals and memorial services he had attended, he pursed his lips, hardened his gaze, and said, "Too many."
Contact John P. Martin at 215-854-4774, at firstname.lastname@example.org or follow @JPMartinInky on Twitter.
Inquirer staff writers Dan Hardy, Michael Matza, and Robert Moran contributed to this article.