More than 200 people were detained for questioning at a nightclub in the northern Mexican city of Monterrey as part of an investigation into this week’s slayings of nine individuals, although most were later released, officials said.

Dozens more were still being held at the State Investigations Agency, or AEI, headquarters in Monterrey, all workers at the La Eternidad nightclub where the nine people killed by suspected gang members had been before their deaths and which is suspected of being a drug-distribution point.

“A search was carried out (in the wee hours of Friday morning) at La Eternidad because the eight men and the woman killed (early Thursday) were found to have that club’s stamp on their wrists,” Nuevo Leon state Security Council spokesman Jorge Domene said.