(CNN) -- A headless body of a man found in a wooded Michigan park had local police in Michigan wondering how -- and why.
Then, as authorities investigated the bizarre killing and waited outside a suspect's home, the suspect tried to outrun the police -- ending with a car crash and the suspect shooting himself.
In the trunk of that crashed car was the body of a teenager who was eight months pregnant.
Father of pregnant murder victim speaks
In the suspect's home, police found an arsenal -- more than 40 guns, hundreds of rounds of ammunition, dozens of machetes, knives and other weapons.
And authorities say it all began with a sex-for-money Craigslist post, written by the victims.
The FBI has been enlisted to help the Wyoming, Michigan, police department investigate the murders of pregnant 18-year-old Brooke Slocum and her boyfriend, 25-year-old Charles Oppenneer. It was Oppenneer's decapitated body, found in a park last Wednesday in Wyoming, Michigan, that sent police into action.
While investigating Oppenneer's death, police learned that his teenage girlfriend, Slocum, was his last known contact, but she could not be located. A day after Oppenneer's headless body was found, the Wyoming Department of Public Safety named Slocum a missing person, according to a police department news release.
Slocum was in her eighth month of pregnancy when she vanished, according to CNN affiliate WOOD.
While executing a search warrant on Slocum's computer, police found e-mail exchanges with a local man named Brady Oestrike.
The e-mails -- which originated with a Craigslist post written by one of the victims -- detailed plans for the 31-year-old Oestrike to meet the couple at a park -- the park where Oppenneer's body was subsequently found -- and pay them for sex, police said.
Last Thursday, while waiting to obtain a search warrant for Oestrike's residence, investigators saw him leave his home in a car, setting off a police chase.
The chase ended when Oestrike's car crashed into a cement barrier. Police found him at the driver's seat, dead from what appeared to be a self-inflicted gunshot wound to the head, according to the police news release.
When police searched the vehicle, they found Slocum's body in the trunk. An autopsy determined that she had been strangled.
Investigators said they later collected more than 400 pieces of evidence from Oestrike's home, including property belonging to both Oppenneer and Slocum.
They found more than 40 firearms, hundreds of rounds of ammunition, knives and other weapons They found restraints indicating that "Oestrike held the victim, Brooke Slocum, in captivity there for a period of time before he murdered her," Wyoming Police Chief James E. Carmody said at a news conference Monday.
A friend of the suspect, Andrew Weener, told CNN affiliate WOOD that Oestrike was interested in the medieval era and that it would not be uncommon for him to own weapons.
"The truth is that's what the medieval times were all about, so I can see how people would jump to that conclusion if they're not well-versed in history," Weener said.
The FBI is involved in the investigation because of "facts of the case that started to come to light" and "some unique characteristics of the case," David K. Porter, supervisory special agent of the FBI told CNN.
Chief Carmody said that Oestrike often traveled to different states and investigators were sorting through financial and travel records to determine the purpose of his trips -- whether for his job, his church where he was a missionary, or otherwise.
"We want to be able walk away at the end of the day and say that we covered all of the things we need to cover and we're confident that there were no other victims and also no other suspects as well," Carmody said.
Both Carmody and Wyoming Police Capt. Kim Koster confirmed that Craigslist was the initial point of contact for the couple and Oestrike. Representatives of Craigslist did not immediately respond to CNN's request Tuesday for comment.
But as the investigation continues, two families in the Wyoming community some five miles from Grand Rapids are in mourning.
"We are thankful for the thoughts and prayers that we've received and in turn, have the deepest sympathy and sorrow for Brooke's family," the family of Oppenneer said in a statement.