By prosecutors’ account, Jorge Torrez crept into the room of a fellow service member at Joint Base Myer-Henderson Hall and wrapped the young woman’s neck with the power cord of her pink laptop. The Marine had some skill in chokeholds, prosecutors allege, and in this case, his strangling was so adroit that a medical examiner could not find any injuries on the woman.

Prosecutors are seeking the death penalty against Torrez, 25, who is charged with murder in the death of Amanda Jean Snell, 20. But in the first days of his trial in federal district court in Alexandria, it seemed they would face challenges in proving their case.

A military medical examiner told jurors Tuesday that although Snell was found stuffed in a wall locker, he ruled both the cause and manner of her death as undetermined. Extensive testing, he said, revealed no injuries or evidence of sexual assault and suggested a possible alternative theory to homicide: A cardiovascular exam, he said, revealed that Snell, a Navy petty officer, had a heart condition possibly responsible for sudden deaths.

“I have no injury,” Sean Swiatkowski, the medical examiner, said as an assistant U.S. attorney pressed him for details of his findings.

To be sure, the case against Torrez is in the early stages, and federal prosecutors are hardly relying on Swiatkowski as their only cog. Most significant, they say they found Torrez’s semen on a bedsheet in Snell’s room, and Torrez denied ever being there. They also called a medical examiner from Ohio, who reviewed the case at their request, to testify that he thought Snell was asphyxiated.

That medical examiner, coroner Humphrey Germaniuk of Trumbull County, Ohio, testified that he noticed a mark on Snell’s neck that was possibly — although not definitely — from a ligature. He said the broader circumstances of her death led him to believe that she was killed.

“People don’t exactly die and walk into closets,” he said. “This does not pass the sniff test. This, we’re dealing with something nefarious.”

Torrez was an active-duty Marine Corps corporal stationed at Joint Base Myer-Henderson Hall, located in the Arlington area, when the death occurred in July 2009. He lived several doors down from Snell in the base’s Keith Hall barracks, prosecutors said, but it is unclear whether the two knew each other on any level.

Prosecutors allege that Torrez browsed Internet pornography sites depicting rape fantasies and randomly attacked other women after Snell’s death. Torrez is now serving five life sentences plus 168 years at Virginia’s Red Onion State Prison stemming from those attacks. He was convicted and sentenced in 2010.

Torrez was charged in connection with Snell’s death in 2011, and federal prosecutors later decided that they would seek the death penalty. Such a decision is uncommon in the Eastern District Court of Virginia, and jurors rarely impose the punishment.

Most recently, prosecutors wanted the death penalty for three Somali pirates convicted in the fatal shootings of four Americans on a yacht off the coast of Africa, but jurors recommended that they instead be sentenced to life in prison.

The last time a jury in the Eastern District recommended death was in 2009 for David Runyon, who was convicted in a murder-for-hire plot in Newport News.

In the case against Torrez, witnesses this week began building a heart-wrenching narrative of the final hours of Snell’s life. A duty officer saw her sitting in the hallway near a common area on July 11, 2009, browsing her laptop. She had just made plans to meet a friend for brunch at an IHOP later that morning.

Base staffers found her days later after she failed to show up for brunch or work.

Michael Rich, an assistant U.S. attorney, told jurors that Torrez bragged about the killing to a jailhouse informant who was recording the conversation. But Rich acknowledged that some details Torrez described were inconsistent with the physical evidence. He argued that Torrez intentionally lied about specifics to throw off investigators.

Although he acknowledged that the medical examiner could not determine how Snell died, Rich noted that Snell was found stuffed in the locker and that her knees bore scrapes that seemed to come after her death. Rich told jurors that happened because Torrez dragged her.

Swiatkowski, the medical examiner, said that the “unnatural position” in which Snell was found aroused his suspicions but did not convince him that the woman had been killed.

He suggested that someone could have put her body in the locker after finding her dead, but he acknowledged that it also was possible Snell died of asphyxia, even if there were no injuries.

Torrez was discharged from the military in April 2010, a Marine spokesman has said. He is also charged in the 2005 killing of two young girls in his home town of Zion, Ill.

DNA evidence linked him to those slayings and forced prosecutors to drop charges against another man.

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