Probably the most compelling expert testimony that the collapse of WTC 7 -- the third building to collapse on 9/11 -- was the result of a controlled demolition comes from Dutch controlled demolition expert Danny Jowenko. If I remember the complete interview is on youtube and runs about 18 minutes. You can see part I here:
Mr. Jowenko had no prior knowledge that there even was a third building that collapsed on 9/11. Then he was shown video footage of WTC7 collapsing. He immediately recognized that it was a controlled demolition and said so. Then he was told that it was WTC7 and that it collapsed on 9/11. He doesn't believe it at first, but refuses to accept any other explanation for the building's collapse than controlled demolition. Eventually he comes up with his own theory of what may have happened: the owner of WTC7, realizing that there had been some damage to the building, decided to bring in a demolition crew and set the explosives and bring down the building later in the day, in order to collect on the insurance.
We are never told what Mr. Jowenko's opinion was of the first two buildings', WTC1 and WTC2, collapse. My guess is that at the time of the interview Mr. Jowenko thought that the first two buildings were not brought down by controlled demolitions. If Mr. Jowenko had thought that they were brought down by controlled demolitions, I'm sure the interviewer would have included that in the video. I've often wondered what Mr. Jowenko's opinion is of all the additional data that has been offered as evidence that the first two buildings were also brought down by controlled demolitions. Perhaps someday someone will find out.