Well, the US may currently be involved in terrorist strikes against Iran... it's been confirmed that we have special forces operating inside Iran and there have been reports that the CIA has been using Wahhabists in terror attacks on Iranian soil... of course no US officials have admitted or confirmed this, but would you?
http://news.bbc.co.uk/2/hi/americas/4180087.stm
http://www2.irna.ir/en/news/view/line-17/0805176169160723.htm
Iran seems to hold the US responsible for attacks on some diplomats at Baghdad's Iranian embassy, back in May, and I am not willing to discount this as a very real possibility. Here's why...
In the last 5 decades or so, US Gov't has installed and propped up brutal dictators, started (and stopped) revolutions in third-world countries, staged events (like the Gulf of Tonkin) as an excuse to go to war, they have tested chemicals on foreign and domestic populations without their consent (MKULTRA), toppled economies, assassinated political leaders, armed Saddam Hussein with WMD to use against Iran, militarized the Taliban and created Al-Qaeda for the Afghan war with the USSR... does any of this qualify as "engaging in terrorism?" I don't know.
Also, this wasn't really the US, but there were two British SAS men who were caught dressed like arabs running around Iraq pretending to be Al-Qaeda, shooting at coalition forces and Iraqi policemen. Remember that?
http://www.theinsider.org/news/article.asp?id=1556
Psycho Magnet pointed out GLADIO, which was an ongoing intelligence op specifically dedicated to staging false-flag bombings and "terror attacks" throughout Europe, from the 1950's up to at least the 1980's. That was not an exclusively US operation, but the CIA was certainly involved. That's declassified.
And Mossad, the Israeli intelligence agency, those guys are brutal. I won't even go into that. Just know that the CIA is, as you say, very tight with Mossad.
And remember, all this is only what we know about. It's been said that a truly successful black op will never come to public light.
Given our recent history, I'd say the US gov't is capable of just about anything.