Joseph DeFelice, 31, of North Amityville, pictured here, and David...

Joseph DeFelice, 31, of North Amityville, pictured here, and David Newbeck, 33, of Massapequa, have been charged with second-degree murder in connection with the Aug. 23, 2010, death of Mandy Jo Jenkins, who had been DeFelice's girlfriend. Credit: SCPD

A Suffolk homicide detective denied a defense attorney's suggestion Monday that he assumed from the start that a North Amityville man was guilty of helping to kill his girlfriend.

During cross-examination of Det. Thomas Walsh, defense attorney Richard Stafford of Bohemia asked about his interrogation training, which teaches that suspects should be questioned as if they are guilty and steered toward making admissions.

Walsh was the final witness at the second-degree murder trial of Stafford's client, Joseph DeFelice, 34. He is charged with helping another man, David Newbeck, 36, of Massapequa, kill his girlfriend, Mandy Jo Jenkins, 30, in August 2010. Prosecutors say Newbeck wanted Jenkins dead because she refused to withdraw an identity theft complaint against his girlfriend.

Newbeck will be tried later.

During questioning by Stafford before Suffolk County Court Judge John Toomey Jr., Walsh said he was just trying to figure out what happened when he talked to DeFelice, who reported the shooting to police.

"I was interviewing him because he said his girlfriend was murdered and taken away by his friend," Walsh said. "I didn't assume he was guilty when we were talking to him. We didn't know what we had. I didn't even know if we had a crime."

Jenkins' body wasn't found for more than a month.

Prosecutors say DeFelice helped Newbeck by giving him a key to the warehouse where he and Jenkins lived and taking his Rottweiller out so it wouldn't attack Newbeck, knowing that Newbeck was about to kill Jenkins.

Stafford noted that after DeFelice signed a 12-page statement, Walsh gave him and the dog a ride to a nearby motel.

"The dog was nice, wasn't it?" Stafford asked.

"I don't know how nice a dog it was," Walsh replied. "He was in control of his dog."

Outside the jury's presence afterward, Stafford asked Toomey to dismiss the case.

"There's no evidence whatsoever that my client intended for Mandy Jo Jenkins to die," he said. "He didn't want Dave to kill Mandy."

Toomey reserved decision. Jurors will hear closing arguments Tuesday.

Get the latest news and more great videos at NewsdayTV Credit: Newsday

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Get the latest news and more great videos at NewsdayTV Credit: Newsday

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