The investigation into Wood's death at age 43 was reopened in 2011. An autopsy report released this month reclassified her death from accidental drowning to drowning caused by undetermined factors.
Wood, who was married to Wagner, sustained bruises on her body before she went into the water, while on a yacht trip with Wagner and their friend, actor Christopher Walken, 69.
Wagner, 82, who was interviewed by police decades ago, has not agreed to speak to investigators again despite about 10 requests that he do so, the BBC quoted Sheriff's Department Lt. John Corina as saying.
"Most of the people we've talked to were never talked to 30 years ago. We've got a lot of new information," Corina said. "All we can do is collect the facts. We're still trying to collect all the facts."
Wagner's lawyer, Blair Berk, said in a statement to the BBC Wagner has "fully cooperated over the last 30 years in the investigation of the accidental drowning of his wife in 1981."
"He has been interviewed on multiple occasions by the Los Angeles sheriff's department and answered every single question asked of him by detectives during those interviews," Berk said.
Wagner has said in the past he thinks Wood drowned either trying to tie up the yacht's dinghy, which had come loose, or attempting to take the small vessel back to shore after he and Walken got into a heated argument. Wagner has always denied any wrongdoing in his wife's death.