From the IDF blog: Specific Flotilla Passengers Linked to Al-Qaeda, Hamas and Other Terror Organizations, 6 June 2010. Two of the five names listed are Americans. More here at Haaretz.
Also, Iran's Revolutionary Guard has offered to provide a military escort to cargo ships trying to break Israel's blockade of Gaza, which has been in place since 2007 when Hamas violently took over the Gaza Strip. Since 1988, Hamas has called for the destruction of Israel for Islam, which is stated in their written charter.
Update 6/7/10: Al Jazeera interview with Ken O'Keefe, one of the two American (if you can call him one). Does he have a tattoo around his neck, or what?? How odd. Anyway, this is one angry man.
On his FB page called Aloah Palestine, he claimed America is the greatest terrorist!
"In response to the Israeli governments claims that I, Ken O'Keefe, am an operative of Hamas who intended to train terrorists in Gaza, I make a counter charge; Israel and the United States, along with their British partner, are the greatest terrorists of our time. Selling, distributing and using more weapons of mass destruction than all other nations combined. When I speak of terrorism I am using the Black's Law Dictionary definition of the word, with that definition it is clear that the United States is the number one terrorist of the 20th and 21st centuries."
He lives in London.
Update 6/8/10: It seems O'Keefe denounced his American citizenship which he talked about in an interview on Al Jazeera in 2008 (its starts off in French but he speaks in English).
"I charge the United States with being the number 1 terrorist in the world..."