Navek, 39, had dated Givens for about four months, including at one point while also living part-time with a fiancée in Williamstown, officials said. The mother of the woman he lived with in Williamstown described her as his longtime girlfriend.
Investigators believed the killings were likely connected, the Camden County Prosecutor's Office said.
It's unclear where Navek and Givens met, but the two worked at the Womack Army Medical Center at Fort Bragg, authorities said. She was a medical-support assistant; he was a physician assigned to the center's acute-illness clinic, a spokeswoman for the medical center said.
A captain in the Army Reserve Medical Command, Navek began his military service in October 2001, according to 6ABC. He served Stateside from February to July 2011 as part of Operation Enduring Freedom in Afghanistan, it said.
A spokeswoman at the National Personnel Records Center couldn't be reached to confirm Navek's service record.
According to Womack, he was activated and assigned to the medical center in September.
Navek and Givens went to visit his family in South Philadelphia for the Easter holiday and returned to North Carolina on Sunday, the prosecutor's office said.
It's unclear when Navek left North Carolina again but relatives said he was back in Philadelphia by 8 p.m. on Tuesday, when he left his Mercedes-Benz with them, saying the car was having problems, the prosecutor's office said.
He borrowed a relative's Nissan and drove to Houshmandpour's house in Voorhees, where he ambushed his former colleague at 7:30 a.m. the next day.
Navek last worked at Virtua more than a year ago. The hospital declined to say under what circumstances he left.
Though Navek's life seemed to be unraveling in recent months, Houshmandpour's was moving in a different direction.
Houshmandpour's family, which is of the Baha'i faith, fled religious persecution in Iran about 15 years ago. A married father of a 20-month-old toddler, Houshmandpour was months away from finishing his residency and was mulling a job offer in South Carolina, his sister Parisa said.
She said her sister-in-law was close to finishing an advanced degree in public health at the University of North Carolina in Chapel Hill and had been admitted to a postdoctoral program at Harvard, so the family was also considering moving to Boston.
On Wednesday, Navek waited outside Houshmandpour's home in the Club at Main Street development. He shot Houshmandpour as the man pulled his silver Audi out of a parking spot.
Navek killed himself when police tried to capture him on nearby Centennial Boulevard. A SIG Sauer handgun and at least 11 rounds of .357 ammunition were found in the Nissan, authorities said.
Later that day, investigators from the Camden County Prosecutor's Office visited Navek's family in South Philadelphia. They searched the Mercedes after learning he had left it there. Inside, they found a bag with personal items belonging to Givens, said Jason Laughlin, a spokesman.
At that point, investigators didn't know Givens was already dead.
The Cumberland County Sheriff's Office in North Carolina and the Camden County Prosecutor's Office began to connect some dots.
"This case demonstrates the perseverance of our investigators. Unlike most of the cases they work on, this isn't something we're going to be able to prosecute," Laughlin said. "But they still kept digging."
Givens' absence from work prompted a coworker to call police, and around 12:30 p.m. Wednesday, officers found her body.
Authorities are awaiting autopsy results to determine when Givens was killed and are investigating whether Navek's gun was used.
Navek grew up in South Philadelphia and worked as a paramedic for four years until February 2000, according to a city spokesman. In 2010, he got his medical license in Wisconsin, records show.
On Wednesday, a relative who identified himself only as an in-law said: "It's just hard to grasp it . . . just in a grieving process right now."
Weeks earlier, Navek was cited in a police report for firing two shots into a basement wall at his Williamstown home, where he then lived with his fiancée and toddler son.
The fiancée told police in March that Navek had threatened her and fired the shots in February. She did not request a restraining order and no charges were filed, the prosecutor's office said.
Navek's neighbors said the woman moved out about a month ago amid police presence. Monroe Township authorities said they were called to Navek's home twice in the last month. Authorities declined to provide more details.
Navek believed Houshmandpour was responsible for his dismissal from the residency program, according to Farogh Mozaffari, the mother of the woman Navek lived with in Williamstown.
Mozaffari said her daughter, also a resident at Virtua, broke up with Navek after finding out he had a girlfriend in North Carolina. Her daughter and Navek had been together about five years, she said.
Contact Darran Simon
at 856-779-3829, dsimon @phillynews.com, or follow @darransimon on Twitter.
Inquirer staff writers Mike Newall and Edward Colimore contributed to this article, which also contains information from the Associated Press.