Explosion

Unknowingly that there are more plenty of those living dead on the second floor.

After Jack made sure he shot those living dead upstairs he reload his AR-15 and his pistol whom he uses earlier instead. He was doing that while his making his way downstairs following the others. Jack quickly move when he suddenly heard a shout from downstairs.

Margaret was the one who shouted when she saw some living dead downstairs and there are lots of them getting near to where they are. Margaret hold the two kids tight while screaming in fear.