Six people, including a police officer and three bystanders, were killed in a furious gun battle Tuesday that filled the streets of Jersey City with the sound of heavy fire for hours, authorities said. The dead included the two gunmen, Jersey City Police Chief Michael Kelly said. The slain officer, Detective Joseph Seals, 40, was credited by his superiors with having led the department in the number of illegal guns removed from the streets in recent years, and might have been trying to stop an incident involving such weapons when he was cut down by gunfire. Two other officers were wounded but were later released from the hospital, authorities said.