Maggie Murdaugh, 52, and her son Paul, 22, were killed at the 1,770-acre estate after Mr Murdaugh called her and asked her to go and see his ill 81-year-old father.
Ms Murdaugh was living separately from her husband at their beach house on Edisto Island and said she would meet him at the hospital. But law enforcement sources told People that he insisted they meet at the compound for her to follow him to the hospital.
As she drove to the house to meet him, she texted a friend and told them that his behavior was “fishy”, the source told the magazine.
“He’s up to something,” she allegedly wrote to the friend.
Authorities say she left her car running when she arrived at the estate and walked to the dog kennels, where her son was. It was there that they were found gunned down, according to investigators.
Ms Murdaugh was shot several times, including once in the back, with ammunition from an AR15-style rifle while her son was shot in the head and chest from close range with a shotgun.
Alex Murdaugh, 54, claimed that he found their bodies when he returned from seeing his father at the hospital.
But the law enforcement sources told People that blood spatter allegedly found on his clothes indicates that he was at the scene at the time of the killings.
He was charged on Thursday with two counts of murder and two counts of possession of a weapon during the commission of a violent crime and will be arraigned next week.
His lawyers have maintained his innocence throughout the bizarre case, which also included him allegedly hiring a hitman to kill him.
“Alex wants his family, friends and everyone to know that he did not have anything to do with the murders of Maggie and Paul. He loved them more than anything in the world,” his legal team said in a statement.