A recent terrorist trial, and conviction, of an Islamic terrorists in New York City brought out the extent to which police have infiltrated Moslem communities in order to uncover terrorist plots. While most of this counter-terrorist activity within Moslem communities is kept secret, enough information has leaked out to make it clear that it's no accident that the United States has not suffered another terrorist attack since September 11, 2001.
There is an extensive informant network within Moslem communities all over the United States. The FBI was pleasantly surprised right after September 11, 2001, by the number of calls they got from American Moslems, reporting suspicious events in their communities, or volunteering to keep an eye on things. It turned out that there was a substantial number of American Moslems, most of them recent immigrants (legal and illegal) that were pro al Qaeda. These attitudes had been causing distress among American Moslems since the 1990s. There had been violence, and even some murders, as the Islamic radicals tried to take control of Mosques, and other Moslem immigrant organizations. While Arabs have been migrating to the United States for over a century, until the 1960s, most of those migrants were Arab Christians, or Jews. Once Israel was founded, the Middle Eastern Jews generally went to Israel, while Arab Christians continued to head for America. Then came the large waves of Arab Moslems. The established Arab-Americans did not have close ties with the growing number of Arab Moslem migrants. There were some pan-Arab organizations, but the Arab Moslems tended to keep to themselves. This was no surprise to the Arab-American Christians, as their ancestors had come to American to escape persecution by Moslems back home. Old animosities die hard.
But many of the Moslem Arab-Americans took to the idea of "becoming Americans," without leaving behind all their cultural baggage from the old country. It was these first and second generation Arab-Americans, as well as Moslem migrants from other parts of the world, that provided the FBI with information about what was going on inside the hundreds of Moslem communities. While there have been some arrests, and prosecutions of American Moslems for terrorist activity, many more pro-terrorist groups simply dissolved when it became known, via some official visits, or gossip, that the feds knew what was going on in the community. The prosecutions that did take place around the country, like the recent one in New York City, almost always resulted in convictions and long sentences. Suddenly, it was no longer a game to sit around and pretend you were going to be a "holy warrior" and bring death and destruction to the Great Satan (the U.S.). Few of the informants within the communities were revealed, and those that tried to ferret them out, or persecute suspected informants, found themselves the subject of police investigation and hostility from the Moslem community. This had a strong deterrent effect on Islamic militants trying to create "liberated zones" in the United States. This was a major difference with the situation in Europe, where many Moslem communities are very pro-terrorist.
But there are still many within the Moslem community who want to carry out terrorist attacks. These people have learned to be careful what they say, and who they talk to. They have become learned in how the American judicial system works, and don't give the FBI an excuse to make arrests, or launch aggressive investigations. For example, police in some parts of the country believe that a few of the "self-defense" groups formed in Mosques have motives beyond protecting the Mosque from anti-Moslem terrorists.
Another good source of intelligence on America based terrorists has been the thousands of terrorist suspects arrested overseas. Many of these men have become cooperative. For example, of the seven different detention centers at Guantanamo Bay, one of them is set aside for "cooperatives." These men get much better treatment, and protection from the hard core terrorists who still comprise most of the other inmates. Most cooperatives aren't sure if they can go home now, because word of their cooperation could get them killed once they are outside Guantanamo Bay. But other resettlement options can be offered, depending on how cooperative the cooperatives can be. The result is often names of pro-terrorist people who have immigrated to the United States, or those in various parts of the world who know something about who is who (terrorist wise) in the United States.
As successful as the informant program has been, it only has to fail once to get a lot of Americans killed, and give al Qaeda a tremendous morale boost. The terrorists know that, so they keep trying.