DNA evidence on a sheath to a small sword left near the body of a slain 12-year-old honor student helped lead Montgomery County police to a suspect in the killing, according to charging documents filed late Tuesday.

On Tuesday, police arrested David R. Hang, 42, of the 17100-block of Queen Victoria Court in Gaithersburg and charged him with first-degree murder in the May killing of Jessica Nguyen. The girl was stabbed to death in the basement of her Gaithersburg home.

Hang is the estranged husband of Jessica’s mother, according to court papers.

Jessica lived with at least five members of her extended family in a townhouse that was part of a new housing development about a mile north of Gaithersburg City Hall.

On May 31, Jessica and her sister arrived home from school about 3 p.m., police said. Jessica’s sister went to her bedroom to do homework and fell asleep. About 7:30 p.m., an aunt asked the sister to find Jessica.

Jessica’s sister looked around the house and eventually found her in the basement “unresponsive and bloody.” There was a sheath to a small sword near her body, but no weapon.

The killing stunned neighbors and those who knew Jessica, a bright and friendly girl who was remembered warmly by her neighbors and classmates at Gaithersburg Middle School.

After her slaying, police launched a broad and methodical investigation, reaching out to anyone Jessica had come in contact with in the days and weeks before her death.

Police swabbed the sheath for DNA and tested it against everyone who lived in the townhouse. None of them matched, according to arrest records. DNA tests later showed that Hang was “included as the major contributor” to the genetic material found on the sheath.

It was unclear Tuesday whether he had retained a defense attorney.

Police called a news conference for Wednesday morning.

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