A forensic specialist testified Tuesday that he believes a Rhode Island man accused of killing his wife while scuba diving off the British Virgin Islands wrestled her from behind and shut off her air supply.

The dive accident specialist, Craig Jenni, who gave similar testimony in a 2006 Rhode Island civil trial that found David Swain responsible for his wife's drowning, testified as the trial entered its second week in the British Caribbean islands' capital, Tortola.

"My impression is that David Swain swam up behind her and held her in a position to turn off her air, thereby not allowing her to breath from either of her air sources," said Jenni, owner of Dive and Marine Consultants International, a Boca Raton, Florida-based company.

SLIDESHOW: Scuba Diving Murder Trial

Jenni theorized that Swain, a former dive shop owner, also prevented his wife, Shelley Tyre, 46, from inflating her buoyancy compensator, a vest that can be automatically inflated to help control a diver's ability to float or sink. He also said that Tyre's snorkel, found missing a mouthpiece, had to have been "forcefully removed."

The 1999 drowning was initially ruled an accident. But authorities in the British Virgin Islands later charged Swain with murder after the 2006 civil trial in his home state found him responsible. He was extradited to Tortola the following year and has been in jail since.

Prosecutors contend Swain, 53, killed his wife so he could pursue a romance with another woman.

Facing a maximum penalty of life in prison if convicted, Swain shook his head slightly a few times during Jenni's testimony. Swain maintains his innocence and his defense lawyers have said they will show the drowning was a "tragic accident."

During cross examination, defense attorney Hayden St. Claire-Douglass argued that Jenni's testimony was biased because he had been hired by the dead woman's parents, receiving roughly $27,000 for investigative work he did for Swain's civil trial.

When the defense lawyer asked if Tyre, an experienced diver, could have panicked underwater, Jennie responded: "I think it's an absolute possibility after she was attacked by David Swain."

The defense also went over Tyre's dive journal, highlighting instances when she described trouble during dives, including difficulty managing her weight belt. In one entry, she wrote that "this weight issue is really my panic button."

Another witness, Bill Oliver, a scuba product developer who studied Tyre's equipment, concluded her mask was torn off. He also said somebody had to "jab" her fin to have it embed in the sandy bottom as it was found.