The Rotunda is a 25-storey office block that housed the "Mulberry Bush" pub on its lower two floors. Within minutes of the warning, police arrived and began checking the upper floors of the Rotunda, but they did not have sufficient time to clear the crowded pub at street level. At 20:17, six minutes after the first telephone warning had been delivered to the "Birmingham Post", the bomb—which had been concealed inside either a duffel bag or briefcase located close to the rear entrance to the premises—exploded, devastating the pub. The explosion blew a 40-inch (100 cm) crater in the concrete floor, collapsing part of the roof and trapping many casualties beneath girders and concrete blocks. Many buildings near the Rotunda were also damaged, and passersby in the street were struck by flying glass from shattered windows. Several of the fatalities were killed outright, including two youths who had been walking past the premises at the moment of the explosion.