Megan Hargan, 35, has been charged with murder in the deaths of her mother and her sister in 2017 in McLean, Va., police said. (Courtesy of Fairfax County Police)

A woman who allegedly killed her mother and sister in their McLean, Va., home and staged the scene to look like a murder-suicide was arrested Friday after a 16-month investigation, Fairfax County police said.

Megan Hargan, 35, is charged with two counts of first-degree murder and other crimes in the July 2017 fatal shooting of her mother, Pamela Denise Hansen Hargan, 63, and sister Helen Lorena Hargan, 23, in their home in the 6700 block of Dean Drive. Megan Hargan, who was indicted by a grand jury Thursday, was taken into custody after a traffic stop in West Virginia, where she had been living, police said.

“Tragedy struck in July 2017 here in our community,” Major Ed O’Carroll of the Fairfax police said at a news briefing. “Today, justice prevails.”

The two victims were found dead in the house on July 14 that year, one in an upstairs bedroom, the other in a laundry room, police said in a court document. Both women had suffered gunshot wounds. A firearm was located next to the woman in the bedroom, and shell casings were found beside the body in the laundry room.

Megan Hargan was living at her mother’s home at the time of the killings.

Although Fairfax police media officials told reporters at the time the incident was an apparent murder-suicide, O’Carroll said Friday “detectives determined early in the investigation that the scene was staged.” Asked how long it took for detectives to rule out a murder-suicide, he said, “not very long at all” and “very, very soon.”

[Apparent murder-suicide leaves 2 women dead in McLean]

As for why the initial public report of a murder-suicide was not corrected by police media officials, Carroll said, “We always try to inform our community.” But he added, “Investigative strategy . . . is of the utmost importance,” suggesting detectives did not want the killer to know the ruse had failed.

Pamela Hargan had worked for Lockheed Martin, the aerospace contractor, for about 25 years and more recently at SAIC, another large government contractor. Another of her daughter’s, reached by phone Friday, declined to comment.

According to a search warrant application filed in court, the bodies were discovered by patrol officers after a man who called 911 said his girlfriend had called him to say her mother had been killed. O’Carroll said Friday the caller was not certain of the exact address, and it was initially hard for officers to figure out where to go.

O’Carroll declined to discuss specifics of the investigation, but said, “Megan Hargan attempted fraudulent money transfers from her mothers [bank] account on the day of the murders as well as the day before.” Megan Hargan was still in police custody in West Virginia Friday and was unclear whether she had retained a lawyer.

A detective wrote in a search warrant application after the killings that the investigation “revealed that the scene may have been altered and staged by the suspect.”

The search warrant also referred to Megan Hargan’s alleged attempts to make fraudulent wire transfers from her mother’s bank account. The document does not say how much money was involved. The search warrant says Megan Hargan had access to her mother’s passwords.

During a search of the Dean Drive home a week after the killing, detectives discovered a laptop belonging to Pamela Hargan and learned documents related to the fraudulent wire transfers had disappeared, according to the search warrant. Police said they recovered cartridges from the master bedroom, the living room floor and a kitchen table, as well as other items, including a shirt from the floor of a bedroom, an iPad and an iPhone.

Dana Hedgpeth contributed to this report.