“See something? Say something.” Easier said than done.

There is a fundamental contradiction at the heart of daily American life. It’s one that undermines the role that citizen sentinels are obliged to serve in the collective effort to combat low-tech aspiring terrorists who may be operating independent of a foreign or domestic extremist network. At once, Americans are supposed to be vigilant and unafraid to express to authorities their fears when they believe someone is behaving abnormally and may represent a threat. At the same time, however, society frowns on those who are perceived as judgmental.

Since social pressures to avoid being seen as hypercritical, paranoid, or—worst of all—bigoted are acute, and the rewards for keeping an eye out for the next terrorist plot are virtually non-existent, the vigilant are often inclined to keep their concerns to themselves. While the public is appropriately skeptical of the likelihood that their eccentric coworker may be a member of the local sleeper cell, there are downsides to this phenomenon. Notable among them is that a series of mass casualty attacks that might have been prevented were not.

Rarely do the potentially violent and disturbed carry out depraved acts of mass brutality without some forewarning. There are always signs. The aspiring jihadist Omar Mateen, the gunman who executed at least 49 people in Orlando on Sunday, was no exception.

Mateen’s former co-worker who now serves his community as a police officer, Dan Gilroy, described him as an “unhinged and unstable” individual who frequently talked about his desire to kill. According to Gilroy, Mateen was also obsessively homophobic. When Gilroy declined to share the would-be murderer’s anti-social views, Mateen became similarly obsessed with him. Gilroy said he would receive up to 15 harassing phone calls from Mateen per day, and twice as many text messages of a similar nature. Gilroy complained to his employer on several occasions about Mateen’s behavior, but the firm declined to do anything about it because—he contended—the employee in question was Muslim. Rather than endure further abuse and without any recourse, Gilroy quit his job.

If you were to ask Mateen’s ex-wife, who claims she was subjected to frequent verbal harassment and beatings before separating from her husband, this mass killer was a ticking time bomb. Others, however, have a more favorable view of Mateen, including a neighbor who found him both positive and chivalrous. Quiet, friendly, kept to himself; these are just a few of the clichéd descriptions of mass murderers provided to reporters by their acquaintances. Their close associates, however, always seem to know the killer differently. Often, they keep their realizations to themselves.

“My son said that he shared [ISIS leader Abu Bakr] al-Baghdadi’s ideology and supported the creation of the Islamic State,” confessed San Bernardino terrorist Syed Farook’s father in an interview with an Italian newspaper. “He was also obsessed with Israel.” Farook’s neighbors became suspicious of the murderous couple when they began receiving an excessive number of packages, but those witnesses didn’t report their suspicions to authorities out of the fear that they would be accused of racial profiling.

Before executing nine African-American parishioners at a South Carolina church in 2015, there were signs that Dylann Roof might be a problem. “He was big into segregation and other stuff,” said Roof’s 21-year-old roommate of one year, Dalton Tyler. “He said he wanted to start a civil war. He said he was going to do something like that and then kill himself.” The security staff at Roof’s local mall had distributed pictures of the future killer after he reportedly rattled employees by loitering and asking them leading questions about staffing and operating hours.

Adam Lanza, the mass murderer who slew a classroom full of young children in Connecticut in 2012 was, by most accounts, clinically disturbed. “Later in life, when he was spending most of his time isolated in his room, Lanza connected online with a community of people interested in mass murder,” Newsweek reported. Increasingly concerned about his penchant for violence, Lanza’s mother tried too late to restrict her son’s access to their family’s firearms. It was a half-measure, but a private one. On the day of his rampage, she became his first victim.

Before his terroristic assault on a Colorado movie theater, James Holmes was terrifying his campus psychiatrist. According to court documents, Holmes’ murderous impulses so disturbed Dr. Lynne Fenton that she confessed her fears to public safety officers on the campus of the University of Colorado, Denver, where her patient was a student. Holmes “may be shifting insidiously into a frank psychotic disorder such as schizophrenia,” Fenton wrote. She stopped counseling him after Holmes began sending her what she described as “threatening” text messages. But the school declined to inform law enforcement or even to restrict Holmes’ campus access after he quit his doctoral program.

Jared Lee Loughner’s classmates describe him as “incoherent” and possessed of a penchant for staring at them intensely or saying inappropriate things. After reading a provocative poem in class, according to a campus police report, Loughner made a bizarre contextual leap and began raving about “abortion, wars, killing people, and “why don’t we just strap bombs to babies.” In an interview, Loughner’s friend described his unease with his longtime associate who he watched descend into madness and nihilism. Loughner went on to shoot and kill 6 people, and to gravely wound Representative Gabrielle Giffords.

Nine of Major Nidal Hassan’s colleagues were disciplined for failing to flag him as a threat following his 2009 Fort Hood rampage, in which he killed 13 and wounded 43 others. They can hardly be faulted for excessive caution considering the penalties for exhibiting what could be considered Islamophobic sentiments in the armed services. “There were definitely clear indications that Hasan’s loyalties were not with America,” said a former classmate of Hassan’s. “They don’t want to say anything because it would be considered questioning somebody’s religious belief, or they’re afraid of an equal opportunity lawsuit.”

“This is a tragedy that’s about a failure of imagination,” recalled author Lucinda Roy when reflecting on the massacre on Virginia Tech’s campus in 2007. Before he killed 27 of his classmates, Seung-Hui Cho’s abnormal behavior made them fear him. “Roy contacted four different departments on campus, including the counseling center and university police,” CBS News reported. Because she was requesting a student receive involuntary counseling, her requests were ignored. “[T]hat’s against Virginia Tech policy as it is at several schools across the country unless it’s an emergency,” the report added, “and administrators claim Roy did not indicate it was an emergency situation.”

The common thread among suspects in these mass shootings and terroristic incidents is not merely that they had mental health issues and an attraction to extremist political ideologies. In each case, the concerned people in those killers’ lives failed to speak up or their warnings were dismissed when they did. For all legitimate concerns regarding the allure of political extremism and the ubiquity of deadly weapons, few seem concerned about the nearly canonical tenets of non-judgmentalism. A cultural proscription on appearing to be prying or condemnatory has its drawbacks; one of them is that people who “see something” often don’t “say something,” or they are ignored when they do.

If America’s preventative approach to terrorism is to deputize its citizens as members of a national neighborhood watch, authorities had better be prepared to listen to them and to act when they see something of concern.

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