Why did the government quickly remove the site of the WTC "terrorist" site evidence without an investigation?

Within months there wasn't a trace of the crash site, body remains or building material left at the WTC. The government closed it off to anyone who wanted to get fragments for investigation. Body remains and all were put into trucks and incinerated, the steel was shipped to a junkyard in China and sold as scrap metal. A few years later after some of the widows of the dead protested and picketed several congressman to do an investigation the government finally gave in and what we got was the laughable 911 commission report. Even a single car accident that kills 1 person gets an investigation. The freeway can be closed down for hours with dozens of crew on site from detectives and police and firemen and engineers and coroner etc trying to determine the details about it, yet at both WTC and pentagon site no investigation was conducted and all the evidence very quickly destroyed. Why? What were they (trying) to hide?