Fresno almost certainly is plagued by several serial arsonists, the city’s highest-ranking firefighter said. Chief Kerri Donis said fire and police investigators are studying 21 fires that have occurred since May. "Some show some common denominators, others not so much," Donis said Tuesday morning after her department’s budget hearing. "We think we have several central arsonists in three or four different areas. We are monitoring that very closely and we have some things in place that may happen next." Several of the fires are giving downtown a burned-out look.