Fifteen young people of hispanic heritage -- including six with known connections to gangs such as MS-13 -- are missing in Nassau County, authorities announced Thursday.

Police revealed the number of missing teens and young adults Thursday amid a massive search operation by local authorities and the FBI's gang task force in the woods and marshes throughout the county after finding the body of a 16-year-old boy and human remains in the county earlier this week. 

It's not clear how many of those missing people -- who make up more than a quarter of the 59 missing persons cases in the county -- are feared alive or dead, or if they're thought to be victims of the violent MS-13 street gang, which has been blamed for at least 22 killings across Long Island since the beginning of 2016. 

Authorities said that two people who had previously been reported missing were found and returned to their families; one girl was with a gang member when authorities found her. 

Earlier Thursday, police and FBI agents combed the Massapequa Preserve after getting a tip about remains there. In May, authorities found the body of a man who they later said had been lured to the field with promises of marijuana and sex only to be killed by alleged MS-13 gang members hoping to increase their stature in the organization. 

Law enforcement sources told News 4 that members of MS-13 tipped off authorities about the locations of gang killings and dump sites. Searches are expected to continue throughout the area in the coming days. 

Earlier this week, U.S. Attorney General Jeff Sessions said in a speech that the gang has become a top priority for the Justice Department.