RIPOLL/BARCELONA, Spain, Aug 19 (Reuters) - Police were searching on Saturday for the driver of a van that killed 13 people when it ploughed into a crowd in Barcelona and were trying to determine whether two other suspected Islamist militants linked to the attack had died or were at large. The Spanish government said it considered it had dismantled the cell behind Thursday's Barcelona rampage and an attack early on Friday in the Catalan seaside town of Cambrils. Police arrested four people in connection with the attacks Barcelona and Cambrils, where a woman was killed when a car rammed passersby on Friday.