AMATRICE, Italy — Rescue workers acknowledged Friday they might not find any more survivors from Italy’s earthquake as they confronted a new obstacle to their recovery work: a powerful aftershock that damaged two key access bridges to hard-hit Amatrice, threatening to isolate it. Mayor Sergio Pirozzi warned that if new roads weren’t quickly cleared to bypass the damaged ones, Amatrice risked being cut off at a time it needs as many transport options as possible to bring emergency crews in and some of the 281 dead out. “With the aftershocks yesterday but especially this morning the situation has worsened considerably,” Pirozzi told reporters.