What happen to WTC7

What really happened to the third 47 story building that also fell that day afternoon about 5PM. This was World Trade Center 7 and it was the tallest building closest to the victimise WTC1(about 300 feet). What is tricky is that this building was not attached by the plane but collapse why? Could it tallest status being the corse which is half the WTC1. Is any one with answer to this?