explain how it can happen?
From the BBC website on such stuff…
Was WTC7 deliberately demolished by explosives?
In the afternoon of 11 September 2001, World Trade Centre Building 7, a 47 storey office block close by the Twin Towers collapsed without even being hit by the planes.
The building had been evacuated and there were no casualties and with so much else happening that day, its collapse was barely reported.
WTC 7 was home to local offices of the CIA, Department of Defense, the United States Secret Service and the city’s Office of Emergency Management, among others.
Sceptics of the official account, including those at Scholars for 9/11 Truth argue that the building was deliberately destroyed in a controlled demolition, perhaps in order to conceal important information about a pre-9/11 plot by the authorities.
The collapse of WTC has been investigated by FEMA. Their interim report found that when the North Tower collapsed, debris crashed into Building 7.
This was the likely cause of fires which quickly took hold. The sprinkler system did not work effectively because the water main in Vesey Street had been knocked out when the Twin Towers came down.
With the intense fires burning unabated, the steel structure supporting the building was fatally weakened. But the FEMA investigators conceded that this hypothesis had a low probability of occurring.
In their final report, due to be published later in 2007, FEMA is expected to back its original hypothesis substantially – the collapse of WTC7 was accidental, not deliberate.