Five of the priests were Spanish and their killings sparked international outrage.
El Salvador's national police force yesterday said in a Twitter post that the four ex-soldiers were arrested at the behest of Interpol in an operation that began Friday night.
They were identified as Col. Guillermo Alfredo Benavides Moreno and soldiers Antonio Ramiro Avalos Vargas, Angel Perez Vasquez and Tomas Zarpate Castillo.
It's up to El Salvador's Supreme Court to rule on the extraditions.
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Federal Magistrate Judge Kimberly Swank ordered that US Marshals take custody of Inocente Orlando Montano Morales so he can be turned over to Spain, pending final approval by State Department.
The step is largely seen as a formality because lawyers for the diplomatic agency already reviewed the case before turning it over to federal prosecutors.
Montano, 73, has denied involvement in the killings. Court documents say that early on the morning of Nov. 16, 1989, members of the Salvadoran military killed the six priests, their housekeeper and her daughter at a university in the country's capital. The priests had been calling for discussions to end the fighting, with one of them serving as an intermediary between the government and a leftist group.
While two officers served short sentences in El Salvador, Montano and other high-level officials were never charged by authorities there in the priests' killings.