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.

Jack upon hearing the loud scream of a woman, he looked down at the side of the stairs. He was worried that they might got bitten and without any much hesitation he jumped through that stairs to reach the second floor and he was just in time before the attacker could possibly bite Margaret and the two kids. Jack fire his AR-15 successively to shoot the attackers (living dead) , there are only fiv