A long-time suspect in the 1996 disappearance of a California college student has been arrested along with his father.

Paul Flores, 44, was the last person seen with Kristin Smart when she vanished on 25 May 1996 while returning to campus from a party.

For years he has refused to comment under his right not incriminate himself, but on Tuesday he and his 80-year-old father Ruben Flores were detained.

Paul and Ruben Flores (right) have been arrested. Pic: San Luis Obispo Sheriff's Office/Facebook
Image: Paul and Ruben Flores (right) have been arrested. Pic: San Luis Obispo Sheriff’s Office/Facebook

The arrests come after a search last month of Ruben Flores’ home using ground-penetrating radar and dogs found new evidence linked to the suspected killing.

But there is still no sign of the 19-year-old’s body, said San Luis Obispo County Sheriff Ian Parkinson.

Reacting to the arrests, Kristin Smart’s family said: “The knowledge that a father and son, despite our desperate pleas for help, could have withheld this horrible secret for nearly 25 years, denying us the chance to lay our daughter to rest, is an unrelenting and unforgiving pain.”

Kristin Smart and Paul Flores both attended California Polytechnic State University campus, about 190 miles north of LA, when she disappeared.

More from US

Paul Flores was arrested at his home in San Pedro, California Pic: San Luis Obispo County Sheriff's Office
Image: Paul Flores was arrested at his home in San Pedro, California Pic: San Luis Obispo Sheriff’s Office

Mr Flores has been a suspect since the start after two students who were with Ms Smart as she walked home said he appeared and promised to make sure she got back safely.

She was never seen alive again.

The LA Times says that during one interview with police, he appeared agitated and told detectives: “If you are so smart, then tell me where the body is.”

The case has gained momentum in recent years after new witnesses came forward and police got permission to intercept Paul Flores’ phone and text messages, as well as searching his home and those of his family.

A full-time cold case detective was hired, while a podcast called Your Own Backyard attracted millions of downloads and sparked new leads.

Police have also been testing cars owned by Paul Flores and his father in 1996 for DNA and other evidence.

Local news site the Tribune of San Luis Obispo said radar, saws and drills were being used as the search of Ruben Flores’ home continued on Tuesday.

Image: Kristin Smart was last seen at about 2am as she walked back from a party

It said sparks were seen coming from the garage and police were also dismantling a deck at the back of the house.

Sheriff Parkinson – who took over in 2011 – said 40 search warrants had been served over the years and that case evidence ran to three terabytes of data.

However, he admitted there were mistakes in the vital days immediately after the disappearance.

“There really is no hiding the fact that there was mistakes made early on and it made it much more difficult,” he said.

“You know that first 48 hours is pretty critical in a missing person or a homicide.”

Ms Smart wasn’t reported missing for three days and a former dorm mate said police were reluctant to take a report because it was Memorial Day weekend and thought she might just have left campus.

University police and the district attorney initially handled the case and it took a month for sheriff’s investigators to fully get involved.

Paul Flores is being held without bail, while his father is on $250,000 bail and is suspected of being an accessory to murder, The LA Times reported.