• Orlando mass killing fits grim pattern of homegrown terrorism, writes Peter Bergen
• Since 9/11, every attack on U.S. soil was carried out by citizen or legal permanent resident, he writes

Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of the new book "United States of Jihad: Investigating America's Homegrown Terrorists."

(CNN)The mass killing in Orlando this weekend fits a grim pattern: Every lethal terrorist attack in the United States in the past decade and a half has been carried out by American citizens or legal permanent residents, operating either as lone wolves or in pairs, who have no formal connections or training from terrorist organizations such as al Qaeda or ISIS.

Because 9/11 was carried out by 19 Arab foreign-born terrorists, many Americans may think that terrorist attacks in the United States are carried out by foreigners, rather than by U.S. citizens.

But Omar Mateen, who on Sunday carried out the deadliest terrorist attack in the United States since 9/11, is an American citizen who was born in New York to parents who immigrated to the United States from Afghanistan.

In fact, since 9/11, a plot which was directed by al Qaeda from Afghanistan and involved only foreign operatives, the real terror threat in the United States has been American citizens or legal permanent residents such as the Tsarnaev brothers, who carried out the Boston Marathon bombings in 2013, and Nidal Hasan, the U.S. Army major who killed 13 at Fort Hood, Texas, four years earlier.

In other ways, Mateen is also typical of jihadist terrorists in the States. He was on the radar of the FBI as a possible militant, just as Tamerlan Tsarnaev, who was investigated by the bureau because of a tip from the Russian government in 2011 that he might be a militant.

Similarly, Nidal Hasan came to the attention of the FBI a year before he carried out the massacre at Fort Hood because he was in frequent email contact with a militant cleric in Yemen.

Mateen was investigated by the FBI in 2013 for inflammatory comments he made to co-workers and a year later because of his connections to Moner Abu Salha, an al Qaeda recruit who grew up in Vero Beach, Florida, and who died in a suicide attack in Syria in 2014 on behalf of the Nusra Front, al Qaeda's Syrian affiliate.

But in the end, the FBI didn't pursue cases against Mateen, Tsarnaev or Hasan.

Although, according to a U.S. official, Mateen pledged allegiance to ISIS in a 911 call around the time that he was carrying out his assault at the Orlando nightclub, no evidence has emerged that he received training from ISIS in Syria, unlike the terrorists who killed 130 in Paris in November and 32 in Brussels in March.

Based on what we know now, Mateen appears similar to other jihadist terrorists in the States since 9/11. There have been more than 300 jihadist terrorism cases in the United States since the 2001 attacks on the World Trade Center and Pentagon. The perpetrators are not the young hotheads of popular imagination. The individuals in these cases have an average age 28, a third are married and a third have children. Mateen was 29 when he carried out the attack, has been married twice and has a three-year-old son.

The homegrown terror threat poses a knotty problem for U.S. law enforcement, as lone wolves are not communicating via email or on the phone with foreign terrorist organizations, the type of communications that can be intercepted by the U.S. intelligence community. Nor do they have meetings with co-conspirators of the type that can be monitored by the FBI.

The FBI has said that it is mounting 900 investigations of suspected Islamist militants in all 50 states. Complicating the difficulty of detecting a mass killer like Mateen is the fact that he worked for a global security firm and was a licensed security officer in Florida, which may have made him -- like Maj. Nidal Hasan -- less suspicious to law enforcement.

The attack in Orlando reminds us that despite all these FBI investigations, sometimes American terrorists will still succeed in carrying out an attack.