WALTERBORO, S.C. — As investigators tried to solve the double murder of Alex Murdaugh’s wife and son, they became so fixated on the idea that Mr. Murdaugh himself was the killer that they “fabricated” evidence and a dubious theory about his possible motive, the defense said on Thursday at the close of the six-week-long murder trial.

Mr. Murdaugh’s lawyer, Jim Griffin, delivered his closing argument shortly before 12 jurors were expected to start deliberating over a case that has captivated people far beyond the South Carolina Lowcountry region where the crime took place.

Prosecutors have argued that Mr. Murdaugh, the scion of a prominent South Carolina family of lawyers and prosecutors, killed his son with two shotgun blasts and then turned a rifle on his wife at the family’s rural hunting estate on June 7, 2021. They said that Mr. Murdaugh carried out the murders of Maggie Murdaugh, 52, and Paul Murdaugh, 22, in a failed effort to keep his longtime embezzlement of millions of dollars from being exposed.

But Mr. Griffin told jurors on Thursday that the notion that Mr. Murdaugh would try to evade scrutiny of his finances by placing himself in the middle of a murder investigation strained credulity.

“Why, why, why would Alex Murdaugh, on June 7, execute his son Paul and his wife Maggie, who he adored and loved?” Mr. Griffin asked, noting the number of people who knew the Murdaugh family and testified about their loving relationship.

The concluding comments from Mr. Griffin came a day after the lead prosecutor delivered his own closing argument, each side trying to connect the evidentiary dots in their favor after testimony from more than 75 witnesses.

Creighton Waters, the lead prosecutor, noted on Wednesday that Mr. Murdaugh admitted on the witness stand that he had told many lies over the years to cover up his financial misdealings and addiction to painkillers. Mr. Waters urged the jurors to avoid becoming the next ones to believe his lies. “Don’t let him fool you, too,” he said.

Much of the trial has focused on Mr. Murdaugh’s lies, including one in particular that he repeated to police in three interviews after the murders: He claimed to have not been at the family’s dog kennels where the crimes occurred on the night of the murders. At trial, prosecutors played a video taken by Paul Murdaugh that captured Alex Murdaugh’s voice at the scene several minutes before the killings took place.

Mr. Murdaugh made the risky decision to take the witness stand in his own defense last week and said, in tearful testimony, that he lied to the police because he feared he would become a suspect if he acknowledged being at the kennels that night. He said he had been there for a few minutes, but then had left and driven to check on his ailing mother who lived about 15 minutes away. He said he returned about an hour later to find his family dead.

Mr. Griffin addressed Mr. Murdaugh’s initial statements to the police directly on Thursday, saying that the video from the kennels turned out to be the backbone of a case that lacked any other evidence. But as a longtime drug addict, he said, Mr. Murdaugh had become accustomed to telling lies.

“Frankly, he probably wouldn’t be sitting over there if he had not lied,” Mr. Griffin said, pointing to his client, who sat at the defense table in a brown blazer and white shirt, intently watching the proceedings. Mr. Griffin added: “He lied because that’s what addicts do. Addicts lie. He lied because he had a closet full of skeletons.”

Throughout the trial, Mr. Murdaugh’s lawyers have argued that the police were sloppy in their investigation and had focused almost exclusively on Mr. Murdaugh instead of looking for other suspects. Mr. Griffin characterized the police investigation as being driven by the concept that “unless we find somebody else, it’s going to be Alex.”

In his testimony, Mr. Murdaugh said he believed the killings were probably carried out by someone seeking revenge over a drunken boat crash in 2019 — a boat the authorities said was driven by Paul Murdaugh — that resulted in the death of a 19-year-old woman.

Mr. Griffin ticked through problems in the investigation, including that the lead agent, David Owen of the South Carolina Law Enforcement Division, had given false testimony about guns found on the property to the grand jurors who indicted Mr. Murdaugh. He also noted that the police had for months wrongly believed that blood spatter had been found on Mr. Murdaugh’s shirt, when, in fact, more rigorous tests taken later showed no blood.

Prosecutors had used “sleight of hand” tricks to mislead jurors at trial, Mr. Griffin added, for instance by making a big deal about a rain jacket with gunshot residue that was found at Mr. Murdaugh’s mother’s home and suggesting that Mr. Murdaugh, if he thought someone was seeking revenge on his family, did not appear to be worried about the safety of his older son after the murders. In fact, Mr. Griffin noted, witnesses who were asked about the jacket said they had never seen Mr. Murdaugh with it, and body camera video from the night of the crimes captured Mr. Murdaugh asking the police to check on his older son, Buster Murdaugh.

He also criticized the prosecution for its contention that Mr. Murdaugh’s inconsistent comments about the timeline of his movements suggested that he was the killer.

“Can you imagine what he saw?” Mr. Griffins said, choking back tears as he described Mr. Murdaugh returning home to find his wife and son killed. “And it’s evidence of guilt that he doesn’t remember what the sequencing was in that moment? Is that evidence of guilt, or is that evidence of trauma?”

Mr. Griffin’s voice faltered again as he finished his closing argument, which lasted just over two hours, by asking the jury to find Mr. Murdaugh not guilty.

“On behalf of Alex, on behalf of Buster, on behalf of Maggie and on behalf of my friend Paul, I respectfully request that you do not compound a family tragedy with another,” he said.