(CNN) -- Investigators have made three arrests in connection with the theft of a missing police captain's truck and have linked the vehicle to a Sunday night robbery in which one person was shot, Virginia State Police announced Wednesday.
But while detectives have made "a significant amount of progress in a short amount of time," they have not yet found 45-year-old Kevin Quick, who has been missing since Friday night, state police spokeswoman Corinne Geller told reporters.
Two sisters and a brother were arrested Tuesday night at a hotel in the Washington suburb of Alexandria, Geller said.
Daniel Mathis and his sister Mersadies Shelton have been charged with grand larceny in the theft of Quick's 1999 Toyota 4Runner, while the second sister, Shantai Shelton, was picked up on an outstanding warrant from Louisa County, east of Charlottesville, where Sunday night's armed robbery took place, she said.
The 18-year-old Mathis also was charged with robbery, malicious wounding and a variety of weapons-related charges in connection with the Sunday night robbery, but no details of that holdup were released. Mathis and his sisters were being held without bond Wednesday evening, Geller said.
Donald Lowe, chief deputy sheriff in Louisa County, said the Sunday night holdup took place at a rural home. Investigators "were able to capture information" about a vehicle that led to the arrests Tuesday night, but no further details were disclosed.
"We're very limited on what we can actually release at this time," Lowe said.
Quick is an auxiliary police captain in Waynesboro, west of Charlottesville. He was last seen at his mother's house in Afton about 10:15 p.m. Friday, and was reported missing after failing to show up for a brother's birthday party Saturday afternoon.
The 4Runner was captured by surveillance cameras at automatic teller machines twice. The first time was in the town of Fork Union, about 50 miles from Afton, about 11:40 Friday night; the second was in Manassas, on the outskirts of Washington, on Saturday night, Geller said.
Each time, the same two people were in the picture, she said -- but authorities haven't said which, if any, of those arrested were in the image. The 4Runner turned up parked under a carport in Mineral, more than 30 miles northeast of Fork Union, on Monday.
Investigators combed that area on foot Wednesday in hopes of finding some sign of Quick, said Sgt. Brian Edwards, a Waynesboro police spokesman. But that search ended unsuccessfully, Geller said.
Virginia State Police say they suspect foul play in his disappearance, and civic leaders have put up $30,000 in reward money for information that helps find him.
"Capt. Quick is a very outgoing person," Edwards said. "He had a lot of street credibility. He automatically commanded respect."
Fork Union is one of two towns where Quick's silver 1999 Toyota 4Runner has been spotted since he was last seen. The other was a sighting later in Manassas, near Washington -- nearly 100 miles away.
Police have linked two men to Quick's SUV and released a photo of one of them: The man wears a hooded sweatshirt that covers much of his face. Authorities hope the public can help them track him down for questioning.
Quick had planned a big bash Saturday for his brother's 40th birthday -- hiring a DJ, hanging decorations and inviting friends and relatives to join in the fun. But the host never showed, prompting his family to start searching for him.
"When he never showed, that was when people could tell there was something really wrong," Edwards said. "Family was really important to him. He wouldn't just ditch out on the family."
Quick's family is deeply rooted in Waynesboro, a city of about 21,000 residents along the edge of the Appalachian Mountains, and has strong ties to its police department. Kevin Quick's late father joined Waynesboro police's reserve arm in 1983. His eldest son joined him in 1990, ascending the ranks over the subsequent years.
The middle brother, Glenn, is an officer with the U.S. Capitol Police in Washington and has joined the search around Fork Union, Edwards said.
Quick worked in the reserves while working day jobs, including the past 15 years as a production coordinator for the synthetic fabrics company Invista. He was laid off three weeks ago, according to court papers, and given a severance package in what Edwards called an "amicable" separation.
The severance package had not yet been accounted for, according to a police affidavit filed as part of a request for a search warrant. The application asked Quick's bank for records of his transactions since January 1.
Quick had been legally separated from his wife since 2007, but their divorce was not final, Edwards said. He said another woman gave birth to Quick's daughter four months ago, and he was headed to their home in Charlottesville when he disappeared.
He had been living with his mother in Afton and took her to breakfast on Friday -- the last time his credit card had been used, Edwards said. He spent that day prepping for his brother's surprise party, then texted his child's mother to tell her he was on the way. His phone has been turned off since just before 11 p.m. Friday, according to the search warrant paperwork.
When the party's 1 p.m. Saturday start time rolled around and there was no Kevin, his family suspected something was up.
According to Edwards, authorities have already ruled out the possibility that he disappeared of his own accord. He was not involved in any undercover or large-scale operations: His duties involved administrative issues and training, Edwards said.
He was off-duty at the time and had no known adversaries or enemies, but his status as a police officer "does bring into question whether or not he could have been targeted," the spokesman said.
Authorities have "traced every possible route that he could have taken" between the various locations as part of extensive air and ground searches, Geller said.
CNN's Carma Hassan contributed to this report.