When Utah police led 20 searchers and five cadaver dogs onto a property in Salem, Ore., this week, they were acting on suspicions that Susan Cox Powell was buried there – and that the man who buried her was not her late husband, Josh Powell, but rather his brother, a politician and graduate student who later committed suicide.

Susan’s father, Charles Cox – who has worked tirelessly with authorities to find his daughter – shares with PEOPLE that West Valley City, Utah, police believe Josh’s brother, Michael Powell, was an accomplice in her murder.

According to Cox and other sources, detectives believe Josh drove the couple’s Town & Country van hundreds of miles northwest in a snowstorm in December 2009 as his and Susan’s two young sons, Charlie and Braden, slept in the backseat. They believe Josh turned Susan’s body over to Michael, who buried her.

But this week’s search on a woodsy, 180-acre property that was rented by Josh’s relatives at the time of Susan’s disappearance did not yield any human remains and the search was called off Thursday after three days.

Michael Powell “was more involved in the disposal of her body,” says Cox, who has endured years of searches that have come up empty and been notified repeatedly of the discovery of remains that turned out not to be Susan’s.

Cox says the police are at the end of their investigation and might soon discuss their suspicion of Michael’s involvement.

A West Valley City police spokesman, Sgt. Mike Powell (no relation), would neither confirm nor deny that police now believe Michael Powell buried Susan’s body. But he confirms that police may soon reveal more about Susan’s disappearance.

He added that information about the case should only be treated as factual once it’s released by the police department. “Any particular aspect of the investigation not coming from the WVCPD should be examined very cautiously,” Sgt. Powell said.

The Early Investigation

Josh and Michael Powell’s sister, Jennifer Graves, who has publicly shared her suspicions about Josh since early in the case, tells PEOPLE that she always suspected that Michael was at least aware that Josh killed Susan.

Now, after further discussing Michael with other family members, she also suspects he was an accomplice to the murder.

“It’s very possible that he helped Josh, yes,” says Graves, who says the revelation has left her heartbroken. “It makes me sad to think he could have sunk so low. Growing up, Michael was a sweet little kid, very loving and a joy to be around.”

She adds that this latest development helps her understand his possible motivations for taking his own life.

“Perhaps he got wind of something [that the police learned] and decided he couldn’t keep it covered up much longer,” Graves offered. “Or perhaps he was just weighed down with a great deal of guilt that finally caught up with him.”

Early in the case, after Josh claimed that Susan simply took off while he and the boys went on a weekend camping trip in a blizzard, police named him a person of interest. The West Valley City police department was recently forced to return federal drug-enforcement grant money that it improperly spent on the Powell case – money that may have been used to investigate whether Josh poisoned Susan’s pancakes, according to media reports.

During the strange twists and turns in the past 3½ years, police seemed to focus their suspicions mostly on Josh and his father Steven.

Josh is now dead, having killed himself and his sons in an intentional house explosion last year. Steven is serving time for secretly photographing 8- and 9-year-old girls who lived next door. Many images of Susan and journal entries about her also turned up in Steven’s house, which was searched repeatedly after Josh and sons Charlie and Braden moved in.

But there has been little focus publicly on Michael, a Democratic National Convention delegate for President Obama who ran unsuccessfully for a state representative seat in Washington state.

In an email to PEOPLE a month after Susan’s disappearance, Michael, a doctoral candidate in cognitive science at the University of Minnesota, ardently defended his “grief-stricken” brother, complained about the “level of integrity” by the media, and suggested police look outside his family for suspects, particularly at a neighbor of Josh and Susan’s in West Valley City, Utah.

“As someone who is close to Josh, I know he is hurting because he misses his wife, and my hope for him is that people are understanding of that,” Michael Powell told PEOPLE on Jan. 16, 2010.

Powell said he drove 1,000 miles to help Josh after Susan’s disappearance because “I am happy to help a family member in need.”

Two years after Susan’s death, Josh Powell adjusted a $1.5 million insurance policy on his sons to name Michael Powell as a 93 percent beneficiary. At the same time, during custody hearings, police would repeatedly tell Charles Cox that Josh’s arrest was imminent.

In February 2012, Josh Powell, frustrated after losing primary custody of his sons to Susan’s parents, blew up his rental house after the boys were dropped off for a visit, killing all three of them.

Michael Powell’s Suicide

Police began seeing clues to Michael Powell’s involvement fairly early in the case, says Anne Bremner, a high-profile Seattle attorney. Bremner represents Susan’s family as well as the victims in Steve Powell’s voyeurism case. She also has participated in many discussions between police and the Cox family.

Bremner says police learned after Susan disappeared that “Michael had his Ford Taurus towed a hundred miles, and then sold it for salvage value because, police think, he had her body in there. Then he hired militaristic satellite photography people to go look at the wrecking yard to see if his car had been completely destroyed, but it wasn’t.”

She adds, “Cadaver dogs came to the tow yard and only indicated on his car, no one else’s.”

More recently, Bremner says, Utah police repeatedly questioned Michael Powell in Minneapolis. Then, three months ago, Michael Powell jumped from the fifth story of a parking structure, taking his own life. He was 30.

Cox says he and the officers believe Michael Powell “couldn’t live with what he’d done, or the police were closing in.” But Cox also said it was he who prompted the search in Salem, Ore., based on a tip he received from a confidential source.

With this latest search completed, Cox says the West Valley City police tell him there will be no more searches and their investigation is finished. Sgt. Powell says the department will do more if new leads spring up, but “we are nearing the end of the tips and leads that we have to follow up on.”

He adds, “Our focus has been finding Susan, and Susan is still missing. It’s been a long, enduring and difficult investigation.”

But Cox vows “we’ll keep searching” until he finds his daughter.

Bremner says Cox is so convinced that Michael Powell buried Susan in the Salem, Ore., area that he’s organizing a search for the weekend of May 25 on public lands surrounding the Salem property, and he already has recruited more searchers than the number that participated in this week’s effort.

Those interested in participating can call her at (206) 486-1200.