by Joe Fryer

Charges will be filed Tuesday against a 19-year-old Savage, Minnesota man arrested in the killing of a woman who answered his Craigslist ad for a babysitter.

Katherine Ann Olson was found dead in the trunk of her car after she answered the ad for a nanny on the popular website.

The suspect was arrested Friday night.

Prosecutors say formal charges will be filed Tuesday morning and the defendant will make his first court appearance at that time.

The founder of believes that Olson's death is the first killing ever linked to the website.

"I can't recall a single case like it," Craig Newmark said.  "This is the worst that I can recall."

Police believe Olson, 24, was killed after responding to a babysitting ad posted on Craigslist.

Her family said Olson was a very trusting young woman who would take babysitting jobs she found online.

One of those ads took her to Savage Thursday morning.

"It seemed kind-of fishy, but she was just going to check it out and meet them," said Sarah Richter, Katherine's sister.

Police found Olson's purse in a park in Savage early Friday, then discovered her body in the trunk of her car in Burnsville.

"I'd just like to express my condolences," Newmark said by phone from California Sunday. "This is a real tragedy and we're more eager than ever to help deal with the bad guy."

Newmark said Craigslist works closely with authorities to investigate crimes tied to the website.

Those crimes include identity theft cases, scams, robberies and sex crimes.

Craigslist has forensic computer experts that can collect evidence and help authorities locate suspects, Newmark explained.

Newmark added that he gives his phone number to investigators from all around the country, and they are welcome to call him at any time.

Olson's family does not blame Craigslist for what happened.

"My best friend married the guy she met on Craigslist," Sarah Richter said. "It's a good place. But you just got to be really, really careful."

The Olsons hope their story encourages people to be cautious when responding to online ads.

"Taking another person with you never hurts," says Karl Olson, Katherine's brother.

Authorities also recommend meeting in a public place.

It's unclear at this time if Olson met her attacker in a public or private place.

The suspect, a 19-year-old Savage man, remains in custody in Scott County.