Members of the MS-13 street gang stabbed a fellow gang member to death in an Alexandria park last fall to keep him from testifying about the group’s criminal activity, according to documents filed in court Thursday.

Boris Elias Rosa Castro, a 19-year-old who lives in Fairfax, admitted Thursday that he helped kill Eduardo David Chandias Almendarez in Four Mile Run Park behind Cora Kelly School around Nov. 28. Almendarez’s body was found about a week later.

The victim was also known by the name Edwin Almandarez.

Prosecutors say Castro killed Almendarez, 22, to keep him from testifying against a fellow gang member in another criminal case. They said that case was a non-fatal shooting in June 2015 and the shooter has pleaded guilty to unlawful wounding.

Two other alleged gang members, Edwin Alexander Guerrero Umana, 18, of Arlington, and Jonathan Gomez Sorto, 22, of Alexandria, also have been indicted in connection with the killing. Their cases are pending.

In a separate case, Alvaro Saenz Castro, 21, who lives in Alexandria, acknowledged Thursday that he too stabbed a man to death in November and left his body in a local park. Saenz Castro pleaded guilty to killing Jose Luis Perez Ferman, 24, near a jungle gym in popular Beverley Park.

However, although authorities said the killer and his victim have MS-13 ties, they think the slaying of Perez Ferman was motivated not by gang business but by a love triangle involving a 16-year-old girl. She was convicted in juvenile court of destroying evidence in the case, and another teen was charged and is awaiting trial.

Officials have said that recent targeted and brutal killings by MS-13 members are part of an effort by the El Salvador-based gang to rebuild its U.S. operations in the wake of a successful crackdown by law enforcement.