Detectives began re-investigating Wood's death in November 2011. The review resulted in Wood's cause of death being changed to "drowning and other undetermined factors."
Woods, Wagner, actor Christopher Walken and the captain of the boat were onboard the yacht when she wound up in the water. L.A. County Sheriff's Lt. John Corina says Wagner is the only person who was on the yacht who has not spoken to detectives as part of the latest inquiry.
"He is like the missing puzzle piece. He is actually the last person to see her alive," Corina said.
Corina said detectives have tried at least 10 times to interview Wagner but have been refused. At one point, detectives traveled to Colorado to try to speak with Wagner but were unsuccessful.
"Most of the people we've talked to were never talked to 30 years ago," Corina said. "We've got a lot of new information."
Investigators say they have many more questions than they did originally. In the past year, they say boaters who were moored nearby that night have come forward - people who investigators never talked to before.
But Wagner's lawyer says his client has nothing new to add.
"Mr. Wagner has fully cooperated over the last 30 years in the investigation of the accidental drowning of his wife in 1981," said attorney Blair Berk in a statement. "Neither Mr. Wagner nor his daughters have any new information to add to this latest investigation, which was unfortunately prompted by those seeking to exploit and sensationalize the 30th anniversary of the death of his wife and their mother."
Yet investigators say he only talked to them once briefly the night of the drowning. They noted that he has talked to the media, giving conflicting accounts.
"What we think is she went outside and slipped on the swim step and hit her head," Wagner said during an appearance on "Good Morning America" in 2008.
But the coroner found no head trauma. A recently revised autopsy showed bruises and scratches - more questions for Wagner.
The boat captain told investigators about a drunken clash between Wood, Wagner and Walken. He says the fight went on at the back of the boat, and then suddenly went silent.
"He didn't tell the original investigators about any of this. And he hasn't talked to law enforcement and told us about this," Corina said. "He was the only one that was in the cabin with her alone ... and then they came out to the back of the boat just him and her."
The Associated Press contributed to this report.