The criminal investigator and host of In pursuit with John Walsh told Good Morning America that it was clear to him after watching the police bodycam footage of the fighting couple in Utah on August 12 that Ms Petito was in serious danger.
“She was scared to death in that video [...] he probably killed her right after,” he said.
It later emerged that a 911 caller had seen Laundrie “slapping” his fiancé, but this information was not relayed to the police who stopped the couple on the road.
Mr Walsh, who also appeared on CNN, asked: “How is it that that 911 operator … didn’t get that information to those two deputies that stopped ’em?”
He continued: “It just was chilling to see that those cops didn’t get the information that the person who called in was so concerned that he pulled over a tourist and called 911 because he saw this guy beating the heck out of Gabby. And it’s so sad because she might be alive today.”
Mr Walsh added: “He terrorised her not to tell the cops that he was the aggressor, he was the slapper and the puncher … She was terrified.”
He also slammed Brian Laundrie’s family, who he called the “dirty Laundries”, for giving him the time to get rid of potential evidence from his home and the van the couple had been living in.
“This dirtbag shows up at his parent’s home ... he’s there for 10 days and he never mentions where Gabby is ... so he’s got 10 days to scrub the van and throw away anything that's in the house,” he said on GMA.
“I’m praying America gets together and we catch this coward” he added.
The family were due to hold a press conference on Tuesday but it was later called off. The Independent reached out to the Laundrie’s lawyer, Steven Bertolino, to ask if had any comment to make about Mr Walsh’s remarks.
Mr Bertolino responded: “I do not know John Walsh nor do I care about his comments.”