Lead investigators in Alex Murdaugh case break their silence - and reveal the moment they cracked that the legal scion had executed his wife and son

The lead investigators in Alex Murdaugh's double murder trial have revealed the moment that they knew the legal scion had killed his wife and son. 

Breaking their silence for the first time since the prominent South Carolina attorney's trial in March, investigators Brett Dove and David Owen spoke to Dateline NBC, in an episode airing tonight at 9pm, about the discovery of video evidence proving he lied about being at the scene of the crime. 

The evidence in question was a final cell phone video recorded by Murdaugh's son Paul on the night of the murders, in which a voice believed to be the patriarch could be heard at a time when he claimed he was not at the scene. 

'I listened to it three to four times, to make sure I was hearing, because I was in disbelief,' said Dove, as Owen described the moment as 'really exciting.' 

'I can prove Alex was lying to me,' he added, and agreed that was the smoking gun that 'blew the case wide open.' 

Murdaugh was convicted on March 2 of the murders of Paul and his wife Maggie by the kennels at their South Carolina hunting lodge in June 2021. He is now serving life without parole, and is appealing his sentence. 

Murdaugh initially claimed in a frantic 911 call that he had discovered the bodies of Maggie and Paul after returning home from visiting his mother, who was suffering from dementia. 

Placing him at the crime scene became a pivotal aspect to the case for prosecutors, as the state said both Maggie and Paul were shot shortly after 8:49pm, when their 'phones locked forever' within seconds of each other. 

Murdaugh had denied ever going down to the kennels after he and his family had dinner together, and only found their bodies shortly after 10pm when he returned from his mother's home. 

But Paul's video would later place him at the scene, as his best friend Rogan Gibson - who the video was shot for as it was of his dog - testified in court he was '100 percent sure' it was Murdaugh's voice in the infamous clip. 

In never-before-seen footage, police have released their first interview with Gibson early in the investigation where he gave the first indication Murdaugh was lying about his whereabouts on the night of the murders. 

The interview, informally taken in a van, Gibson said he was boarding his chocolate lab dog at the kennels when he received a call from Paul, just two hours before the killings. 

'Paul called me at 8:44pm,' he said, noting that - the same as in Paul's final cell phone video - he heard other people in the background believed to be Alex and Maggie Murdaugh. 

'I heard Ms. Maggie, and believed to be what was Mr. Alex in the background,' he said. 'I can't swear that was Mr. Alex, but I believe that it was Mr. Alex.' 

When asked how well he knew Murdaugh to be able to recognize his voice, Gibson said he was like 'a third son to him.' 

'I'm 99 percent sure that was Mr. Alex talking to me, or talking to them while I was on the phone (with Paul),' he said at the time, later testifying he was '100 percent sure.' 

In his recent interview with NBC Dateline, Owen said the early interview immediately raised red flags about Murdaugh. 

'I was like, 'wait a minute, that's after they had dinner, when Alex told me he stayed in the house while Maggie went to the kennels,' the investigator said. 

Asked why he would believe Gibson's account over Murdaugh's, Owen added: 'The information he gave us was unsolicited. Nobody prompted him to say that.' 

Murdaugh's trial has been dubbed South Carolina's 'trial of the century', which also included shocking interview footage of Murdaugh appearing to 'confess' to the killings, and a disastrous appearance on the stand where he testified in his own defense. 

The killer is now appealing his conviction, and filed a motion for a new trial last month over claims of jury tampering. 

His attorneys filed for an evidentiary hearing on allegations against Colleton County Clerk of Court Rebecca Hill, who has been accused of improperly counseling jurors to disregard his testimony and pressured them to reach a quick verdict. 

'When those facts are proven, the court must grant a new trial,' the motion reads. 

Murdaugh won a victory from South Carolina's Supreme Court on October 17 after it granted a motion to pause his conviction appeal while he seeks a re-trial over the jury tampering claims. 

Murdaugh's bid for freedom is set to be decided by a separate judge to the one who sentenced him, however, as judge Clifton Newman recused himself from any post-trial hearings this week.

His lawyers called for Newman to step down after giving a controversial TV interview, after they previously sought to block the judge from hearing Murdaugh's request for a new trial. 

The lawyers had also wanted the Supreme Court to remove Newman from an upcoming case prosecuting Murdaugh over a string of alleged financial crimes that prosecutors say he committed as his life spiraled out of control before the murders. 

In an interview with NBC News a month after the trial, Newman said he 'felt sorry' for Murdaugh and believed that 'if he had an opportunity to do it over again, he'd never do it.' 

He also said he was not surprised at the spotlight being placed on him during the blockbuster trial. 

'You know, high-profile lawyer. Death of a wife. Death of a child. Accusations of stealing millions of dollars from clients. Allegations of a lawyer hooked on drugs,' Newman said. 'It had all the ingredients for something of major public interest.'