Complex sentence: NIST considered the possibility that 7 WTC was brought down with explosives and concluded that a blast event did not occur, that the "use of thermite (...) to sever columns in 7 WTC on 9/11/01 was unlikely".
Split into simpler sentences: NIST considered the possibility that 7 WTC was brought down with explosives. It concluded that a blast event did not occur and that the "use of thermite (...) to sever columns in 7 WTC on 9/11/01 was unlikely".