Might this be considered circumstantial evidence in response to the claim that bombs could not have been planted unnoticed in the WTC?

GAO: Major Security Flaws at Federal Buildings

The police agency in charge of protecting thousands of federal buildings nationwide has failed to keep bomb-making materials out of several high-security facilities in the past year, according to Congressional testimony provided by Senate aides. In the past year, investigators successfully smuggled bomb-making materials into ten high-security federal buildings, constructed bombs and walked around the buildings undetected, exposing weaknesses in security provided by the Federal Protective Service.

More than one million government employees work in 9,000 facilities guarded by FPS around the country, including at least 350,000 in the Washington region. The revelations come as the Obama administration prepares to reorganize the agency in the coming weeks.