Three runaway teenage girls have been returned home after an investigation that began with Fairfax County police looking into death threats allegedly made by a member of the MS-13 gang.

Police said they are exploring whether the girls were part of a trafficking or prostitution ring.

Fairfax County police charged 10 people last month in the gang-related killing of Damaris A. Reyes Rivas, 15, a runaway from Gaithersburg who was slain in a Springfield park in January. Police said in a search warrant that Reyes Rivas was killed in retaliation for the slaying of an MS-13 member in Prince William County about a week earlier.

Some of those same suspects were also charged in the disappearance of a Springfield 16-year-old, Lizzy Colindres, who left home with her 5-month-old son in mid-January before returning safely in February.

Police do not think the most recent case is related to Reyes Rivas's killing or Colindres’s disappearance.

In the recent case, Fairfax police began investigating 19-year-old Manuel Lopez Aguilar on March 11 after his ex-girlfriend reported that he was threatening her, according to a search warrant filed in Fairfax County Circuit Court.

The ex-girlfriend told authorities that Lopez Aguilar had been making threats since the couple broke up the year before. A detective wrote that the ex-girlfriend showed police text messages, audio messages and photos that Lopez Aguilar sent her via Facebook Messenger. In one, Lopez Aguilar allegedly told the ex-girlfriend that “he has no problem coming after her and killing her.”

The photos included shotgun ammunition, a Oujia board and Lopez Aguilar pointing a handgun while making MS-13 hand signs, according to the search warrant.

The detective wrote that the ex-girlfriend was also sent photos of two girls who were living with Lopez Aguilar and whom the ex-girlfriend suspected were runaways.

Lopez Aguilar was arrested and charged with making death threats, police said. Police said they do not think Lopez Aguilar was involved in trafficking or prostituting of the girls, but the investigation led police to the missing girls.

The first runaway girl, 17, was found in Falls Church on March 10. The second runaway girl, 16, was found with Lopez Aguilar when he was arrested in Falls Church on March 11. A third girl, 14, was found the next day in Maryland. The police did not release the names of the girls because they are juveniles.

Separately, Fairfax police are also investigating the disappearance of two other teens who are believed to be in danger and have connections with gangs. Edvin Escobar Mendez, 17, of Falls Church and Sergio Arita Triminio, 14, of Alexandria disappeared last fall.

Fairfax police are also investigating the case of two bodies that were discovered in Holmes Run Park in the Lincolnia area in early March. Police are exploring whether those deaths are related to MS-13.

Fairfax has seen a 70 percent increase in MS-13 activity from 2015 to 2016, according to police. Detectives said the gang has been reconstituting itself after a major bust in 2014. It is considered the largest and most violent gang in the D.C. area.