(CNN)What organization has mounted the most terrorist plots in the United States since 9/11? The FBI.

As part of its key mission of trying to stop the next terrorist attack, the law enforcement agency has mounted more than 30 plots that were, of course, sting operations in reality.

Peter Bergen

Despite the FBI's efforts, some "homegrown" American terrorists still have managed to carry off lethal attacks in the States in recent years, in places such as San Bernardino, Boston and Fort Hood, Texas.

This has touched off political debate, particularly among Republican candidates, about how to safeguard Americans, including spurious solutions such as shutting off Muslim immigration (Donald Trump) and carpet-bombing ISIS (Sen. Ted Cruz). Neither approach is realistic, not least because lethal attacks by jihadist terrorists in the States since 9/11 have been conducted largely by American citizens, while ISIS is embedded in major cities in Iraq and Syria and so carpet-bombing would kill a great number of civilians.

In fact, the real lessons learned should come from the law enforcement agencies that have studied jihadist terrorists in depth. A very telling indicator of future violence by a terrorist, FBI behavioral analysts have found, is what they term "leakage."

Leakage was first identified by the FBI in 1999 in the context of school shootings, emerging from the observation that a student who was going to do something violent had often intentionally or unintentionally revealed something significant about the impending act, anything from confiding in a friend to making ominous "they'll be sorry" remarks.

Leakage is, in short, when a violent perpetrator signals to people in his circle that he is planning an act of violence.

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What was true of school shootings turned out to be true for terrorist crimes as well.

In an ongoing study of some 80 terrorism cases in the States since 2009, which has not been previously reported, the FBI found that "leakage" happened more than 80% of the time.

Those to whom information was leaked, termed "bystanders," were broken down by the FBI into peers, family members, authority figures and strangers.

FBI analysts found an average of three bystanders per case, and in one case as many as 14.

Some "bystanders" saw radicalization behavior. Others saw actual plotting and planning, such as the accumulation of weapons, self-educating about how to make explosives or preparations to travel overseas for terrorist training.

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FBI analysts were dismayed by how common it was for bystanders to know that a radicalized individual was up to something yet failed to tip off the authorities.

Analysts graphed out the bystanders who were most likely to come forward with information versus those least likely to do so.

Peers were aware of the most concerning information, but they were the least likely to volunteer it. Family members were often aware of both radicalization and planning, but they came forward less often than authority figures such as college professors, supervisors, military commanders or clerics. These figures were reasonably likely to offer information but were more aware of a suspect's radical sympathies than of any actual plotting.

Strangers were the most likely to come forward, which could be helpful. A tip from a clerk at a New Jersey Circuit City -- who in 2006 was asked to make copies of a videotape on which he saw men shooting off weapons and shouting "Allahu Akbar!" -- developed into the case in which a group of six men were convicted for plotting an attack to kill soldiers at the Fort Dix, New Jersey, army base.

However, strangers made up only 5% of the bystanders with useful information about a suspect, which raised an interesting problem for the "If You See Something, Say Something" counterterrorism campaign that is targeting that 5%, rather than the 95% of the peers, family members and authority figures who generally had the most useful information about a militant.

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The "If You See Something, Say Something" campaign also generated quite a number of false positives from suspicions of, say, a Middle Eastern man taking a photo of a bridge.

The importance of the information that a peer can have was underlined by the terrorist attack in San Bernardino on December 2 in which 14 people were killed by the married couple, Syed Rizwan Farook and Tashfeen Malik. Farook's friend, Enrique Marquez, allegedly provided the two semiautomatic rifles that Farook and his wife used in the massacre. Authorities have said Marquez allegedly also knew that Farook was planning to carry out some kind of terrorist attack as early as 2011. Marquez has been charged with a variety of federal crimes for his alleged role and has pleaded not guilty.

The prevalence of leakage in terrorism cases has opened up some potential investigative avenues for the FBI.

As one agent put it, "If you're going to commit a significant act of terrorism, I don't think it's possible to do that without having some leakage someplace. In the future, perhaps we can pick that up either through behavioral study, through technical coverage or through some type of investigative technique."

The lesson of the FBI study of terrorism cases is that the most useful information comes from peers and family members. That's why community outreach to Muslim communities to enlist their help in detecting those who may be becoming militant is the most fruitful approach to dealing with the scourge of terrorism.

This is the opposite approach from painting all Muslim immigrants as potential terrorists.

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