Detectives spent several days trying to determine if the shooting was a homicide or suicide.
"We have gathered enough information now," Los Angeles County Sheriff's Lt. Dave Dolson said. "We are certain it's a murder."
Detectives did not immediately label the death a homicide because no neighbors witnessed the shooting or saw anyone flee the scene.
Sheriff's deputies recovered a single shell casing at the scene, but did not find a gun. However, authorities said it was not unheard of for a weapon to be removed from a body immediately after a suicide.
Tidus and his wife had returned home from her business, Child's Play Toys, shortly before he was shot. His wife reportedly told police he had gone out to retrieve a laptop from his Toyota Prius. Moments later, she found him sprawled in the driveway with a pool of blood around his head.
Tidus was a partner at the law firm Baute & Tidus LLP in downtown Los Angeles. According to his Web site, Tidus handled a wide variety of business litigation and securities cases, as well as professional malpractice. He represented several corporate clients such as Hawthorne Savings, Isuzu Motors of America and New Century Financial.
New Century was one of the country's largest subprime lenders before it failed two years ago. On Monday, three of its top officers were sued by the Securities and Exchange Commission (SEC) and charged with fraud.
Detectives were going over his past legal cases and interviewing his business partners to determine whether his death was linked to his work. They were told some people might have held grudges against Tidus due to his legal work, but Dolson said it was too soon to say if Tidus was the victim of an orchestrated hit.
It was unclear if Tidus had received any threats.
"Regardless of what you call this investigation, and they are now calling it a murder investigation, it will be complete and thorough," said Steve Whitmore, a spokesperson for the Los Angeles County Sheriff's Department.
An autopsy was performed Thursday.
The Associated Press contributed to this report.