People are still gathering information about Omar Mateen, the shooter of the Orlando nightclub. However, Mateen was twice investigated by the FBI over suspected ties to terrorism. An unnamed source quoted in the Daily Beast said, "He's a known quantity," yet there were no further inquiries.

Although it is too early to tell what the domestic political response will be, violent tragedies often generate calls for tighter surveillance policies, such as the Patriot Act following the September 11, 2001 attacks. Following the shooting in San Bernardino, California last year, New Jersey Governor Chris Christie assailed politicians who "voted to take away tools from our intelligence community that permits us to be able to connect the dots" to uncover plots.

Following are five surveillance and security failures under contemporary surveillance policies and practices enacted since September 11, 2001.

1. Nidal Hasan

In 2009, Major Hasan, an army psychiatrist, shot and killed 13 people at Fort Hood, Texas. FBI investigations of Hasan prior to the shootings found that he had been exchanging emails with Anwar al-Awlaki, an Islamic cleric who had already been named on terrorist watch lists with orders to be arrested.

2. Umar Farouk Abdulmutallab

Awlaki's name surfaced again only a few weeks after the Fort Hood shooting as an associate of Abdulmutallab, who attempted to detonate plastic explosives concealed inside his pants. The father of the so-called "Underwear Bomber" approached U.S. officials at the American embassy in Nigeria to warn them about his son's suspected radical views. Abdulmutallab's name was added to a database and was never investigated due to "insufficient derogatory information available."

3. Tsarnaev Brothers

At the 2013 Boston Marathon, Tamerlan and Dzhokhar Tsarnaev discharged explosive pressure cookers that killed three people and injured more than 250 others. Two years prior, Russian security agencies warned the FBI about elder brother Tamerlan's militant associations. The CIA also pushed to have Tamerlan, who died in a shootout with authorities after the bombing, placed in the Terrorist Identities Datamart Environment (TIDE) database.

4. Richard Reid

Months after the September 11, 2001 attacks, Reid attempted to board a plane from Paris to Miami. Although he was initially denied access to the plane because of his bizarre behavior and appearance, Reid boarded the plane the next day with explosives in his shoe. Reid ignited his shoe but it failed to explode. The failed terrorist attack by Reid, who was dubbed the "Shoe Bomber," led to the practice of passengers removing their shoes at airport security. However, ordinary scanners at the time were unable to detect PETN, the explosive chemical used by Reid.

5. Faisal Shahzad

In another of the security failures, Pakistan-born Shahzad parked and abandoned an SUV in New York's Times Square on May 1, 2010. Bystanders noticed the smoking vehicle and alerted local law enforcement. Shahzad tried to board a plane for Dubai two days later, but not before his name was added to the no-fly list. His apprehension on the aircraft was lauded by some as a triumph of the system, but officials failed at least twice to arrest Shahzad which allowed the suspect to board the plane. Then-New York mayor Michael Bloomberg conceded, "We got lucky."