Woman found stuffed in suitcase in Connecticut died of 'homicidal asphyxia': Official Valerie Reyes' body was found in Greenwich, Connecticut in February.

A 24-year-old woman whose body was found stuffed into a suitcase in Greenwich, Connecticut, died of "homicidal asphyxia," or lack of oxygen, Linda Sylvia, spokeswoman for the Office of State Medical Examiner, told ABC News.

It was not clear when she died.

Valerie Reyes' body was found on the side of the road on Feb. 5 with "obvious signs of head trauma," according to a criminal complaint charging her boyfriend, 24-year-old Javier Da Silva, with her death. She was last seen on Jan. 29 and was reported missing by her family the next day, the complaint states.

When Da Silva was interviewed, he "admitted a role" in Reyes' death, Greenwich Police Capt. Robert Berry said on Feb. 12.

Da Silva told investigators that he and Reyes had sex at her home in New Rochelle, New York, on Jan. 29 and that she "fell to the floor and head her head," at some point, according to the complaint.

Da Silva then allegedly put packing tape over her mouth, bound her legs and hands and placed her into the suitcase, which he left in a forest, the document states.

Investigators narrowed in on Da Silva after he used her ATM card multiple times after she was killed, Berry said at the time.

Da Silva was arrested on Feb. 11 in Queens, New York, and was charged with kidnapping resulting in death. He is a citizen of Portugal and entered the U.S. in May 2017 through the Visa Waiver Program. However, he overstayed his visa, according to U.S. Immigration and Customs Enforcement.

Da Silva is being held without bail in federal custody. He was not required to enter a plea when he appeared in a federal magistrate court in White Plains in February.

In April, it was revealed in court documents that Da Silva and the feds might be working towards a "possible disposition of this case." According to the Greenwich Time, the US attorney suggested the negotiations may result in a plea deal and a lawyer for Da Silva did not respond to a request for comment.

If convicted, Da Silva could face the death penalty, according to The Associated Press. ABC News could not immediately reach an attorney for Da Silva.

ABC News' Ahmad Hemingway, Aaron Katersky and Emily Shapiro contributed to this report.