GAINESVILLE, Fla. -

On the 25th anniversary of the disappearance of Tiffany Sessions, Alachua County Sheriff Sadie Darnell says it's "highly, highly probable" that Paul Rowles -- a man who died in prison one year ago doing time for murder, kidnapping and sexual battery -- was responsible for the disappearance of the 20-year-old college student in 1989.

"Come forward if you were involved," Darnell said in a Thursday morning news conference.

"We need the public's help. We are looking for a very small needle in a very big haystack," said Hilary Sessions, Tiffany's mother. "We just need one tip."

The Sheriff's Office invited the media to the site where authorities are digging for new evidence, where they released details of the investigation, case photographs and information regarding the suspect in hopes that disclosing it will result in leads from the public that may help solve the case.

Session's father, Patrick Sessions, a prominent South Florida real estate developer, used his connections with former Miami Dolphins quarterback Dan Marino and other NFL players to gain attention to the case, but his daughter was never found.

Paul Rowles Rowles (shown in Florida Department of Corrections photo) was 64 when he died in February 2013 while in state custody. According to Patrick Sessions, Rowles was in Gainesville working as a pizza delivery man and for a construction company in the city at the same time Tiffany Sessions and another woman whose body was recently discovered lived there.

"He was a stalker. He planned all of his crimes in meticulous detail. He was very detail-oriented, (had obsessive-compulsive disorder)," lead investigator Kevin Allen said. "I'm sure he met her either from a pizza delivery or when he was putting up construction scaffolding at the apartment complex on the walk that she took every day. I'm sure he planned this crime out well in advance. He probably practiced it several times, and when he took Tiffany, he took her to her grave."

The site where investigators are searching is where the body of Beth Foster was found in 1992. It wasn't until 2012 when DNA evidence pinpointed her killer as Rowles.

After Rowles died last year, investigators went through the stuff in his prison cell. They found the date Sessions vanished written in his calendar -- written "#2 2-9-89 #2" -- and they believe the "#2" stands for his second victim.

His first was a woman from Miami whom he was convicted of killing in 1972 but was released nine years later.

"Beth Foster was found in the wooded area behind me," Darnell said. "Her grave is marked with an orange stick in the dirt. We believe this was potentially a dump site also or disposal site for Tiffany. We have dug up this area. We had a major dig a little over a week and a half ago. We're back to finish the effort, hoping to find the remains."

At Thursday's news conference, investigators displayed photo boards of Rowles at various ages, the red Bronco he drove at the time and a timeline of his life, hoping it will trigger someone's memory.

"I know it can be closed," Hilary Sessions said. "We just need that little piece of information that somebody has, and I want to make sure you're not going to get prosecuted or go to trial because Paul Rowles is dead and there's no possibility of a trial."

"This is proof that there are evil men among peaceful people," Gainesville Mayor Ed Braddy said.

Patrick Sessions is hoping that if his daughter's remains are recovered, DNA will point toward a killer, or that Thursday's announcement will jog someone's memory about Tiffany's disappearance.

Jason Sessions, Tiffany's younger brother, said coming back to Gainesville and watching police search for evidence in how she disappeared hasn't been easy.

"This has been tough. Standing here for two weeks watching them excavate this site looking for Tiffany's remains has been a difficult process for us," said Jason Sessions. "It's been very painstaking, it's been long, it's been a difficult process for me for sure. It stirred up a lot of emotions that haven't been that strong within our family in a number of years."

The Alachua County sheriff says it's "highly, highly probable" that Rowles killed Tiffany. Jason Sessions thinks of Rowles the same way he would any other convicted killer.

"Yeah you know, I don't, as I was saying before, I don't necessarily think of this guy any differently than I do the other criminals that we've been investigating. There's a lot of nasty people out there," said Jason Sessions. "There's a lot of people everywhere that do harm to people, especially women. It's a terrifying world out there and you can't live every day in fear, but you have to be cognizant of what's around you."

The lead investigator for the case went to Miami to try to speak with Rowles before he died but it was too late. Rowles was in ICU on a ventilator, dying from lung cancer. He'd been asked about Tiffany Sessions once before after DNA linked him to Foster's murder but he refused to cooperate.