NEW BRAUNFELS, Texas (CNN) -- The defense rested its case Wednesday without calling a single witness in the capital murder trial of David Graham, the former Air Force Academy cadet accused of murdering a teen-age girl with whom he allegedly had a sexual tryst.

Attorneys for both sides are expected to make closing arguments Thursday, before the seven-man, five-woman jury begins deliberations.

Graham, 20, faces up to life in prison if convicted in the December 1995 murder of 16-year-old Adrianne Jones. He is accused of shooting her twice in the head to prove his love for his fiancee, Diane Zamora, after telling Zamora he had sex once with Jones after a high school track meet.

Defense attorney Dan Cogdell had told reporters he expected to present several witnesses, possibly including Graham. He surprised the court when he rested immediately after prosecutors closed their case, following five days of testimony.

"Mr. Graham, your honor, stands on his presumption of innocence and rests," Cogdell told state District Judge Don Leonard.

Zamora refuses to testify

Outside the courtroom, Cogdell explained, "I decided the best way to keep the jury focused on the weakness of the state's case was to rest right behind them."

Zamora, a former midshipman at the Naval Academy, was convicted earlier this year and sentenced to life in prison for her part in Jones' murder.

Earlier Wednesday, she refused to testify. Cogdell said Zamora's testimony would have proven his contention that she alone killed Jones, and Graham was not even present at the murder.

Prosecutors allege that Zamora became angry because Graham had a one-time liaison with Jones and ordered him to shoot Jones after she hit her over the head.

Zamora, brought in from state prison, invoked her Fifth Amendment right not to incriminate herself, and refused to answer questions as she gazed sadly at her former boyfriend.

It was the first time the two had seen each other since a hearing last August to determine if they should face trial separately. Several times, Zamora looked at Graham, who leaned back in his chair and smiled.

An obsessive relationship

Witnesses described their relationship, which started in August 1995 when they were both high school seniors, as obsessive and all-consuming. After high school, when they were at their respective service academies, they wrote each other letters every day.

Zamora's attorney, John Linebarger, who advised her not to testify because she is appealing her life sentence, said Zamora no longer loves Graham. "I think the bloom has fallen off the rose," he said.

The Jones murder went unsolved for months until Zamora told her roommate at the Naval Academy about it in the summer of 1996. The roommate told authorities.

The trial was moved from Fort Worth to New Braunfels, 30 miles northeast of San Antonio, because of pretrial publicity.

Reuters contributed to this report.