Elizabeth Alvarado and Rob Mickens, parents of slain Brentwood High student Nisa Mickens, are still mourning their daughter at their home in Brentwood, N.Y. (Michael Noble Jr. for The Washington Post)

BRENTWOOD, N.Y. — The old minivan appeared near the school on a Tuesday morning, its Illinois plates the only thing out of place in the blue-collar suburbs of central Long Island. But as backpack-toting teenagers passed by on their way to Brentwood High, the van’s doors suddenly swung open.

Out sprang members of the violent street gang MS-13, armed with baseball bats.