An angry outburst at a mosque. The posting of a suspicious YouTube video. A friendship with a shadowy imam.
Those were just some of the signs that Tamerlan Tsarnaev, accused of masterminding the Boston Marathon bombings, had adopted a virulent strain of Islam that led to the deaths of four people and injury of more than 260.
But how else can you tell that someone's religious beliefs have crossed the line? The answer may not be as simple you think, according to scholars who study all brands of religious extremism. The line between good and evil religion is thin, they say, and it's easy to make self-righteous assumptions.
"When it's something we like, we say it's commitment to an idea; when it's something we don't like, we say it's blind obedience," said Douglas Jacobsen, a theology professor at Messiah College in Pennsylvania.
Yet there are ways to tell that a person's faith has drifted into fanaticism if you know what to look and listen for, say scholars who have studied some of history's most horrific cases of religious violence.
"There are a lot of warning signs all around us, but we usually learn about them after a Jim Jones or a David Koresh," said Charles Kimball, author of "When Religion Becomes Evil."
Here are four warning signs:
1. I know the truth, and you don't.
On the morning of July 29, 1994, the Rev. Paul Hill walked up to John Britton outside an abortion clinic in Pensacola, Florida, and shot the doctor to death. Hill was part of a Christian extremist group called the Army of God, which taught that abortion was legalized murder.
Hill's actions were motivated by a claim that virtually all religions espouse: We have the truth that others lack.
Those claims can turn deadly when they become absolute and there is no room for interpretation, Kimball says.
"Absolute claims can quickly move into a justification of violence against someone who rejects that claim," Kimball said. "It's often a short step."
Healthy religions acknowledge that sincere people can disagree about even basic truths, Kimball says.
The history of religion is filled with examples of truths that were once considered beyond questioning but are no longer accepted by all followers: inerrancy of sacred scripture, for example, or the subjugation of women and sanctioning of slavery.
If someone like Hall believes that they know God's truth and they cannot be wrong, watch out, Kimball says.
"Authentic religious truth claims are never as inflexible as zealous adherents insist," he writes in "When Religion Becomes Evil."
Yet there's a flip side to warnings about claiming absolute truth: Much of religion couldn't exist without them, scholars say.
Many of history's greatest religious figures -- Moses, Jesus, the Prophet Mohammed -- all believed that they had discovered some truth, scholars say.
Ordinary people inflamed with a sense of self-righteousness have made the same claim and done good throughout history, says Carl Raschke, a theology professor at the University of Denver in Colorado.
The Protestant Reformation was sparked by an angry German monk who thought he had the truth, Raschke says.
"Martin Luther's disgust at the worldliness of the papacy in the early 1500s inspired him to become a radical revolutionary whose ideas overturned the entire political structure in Europe," Raschke said.
So how do you tell the difference between the healthy claims of absolute truth and the deadly? Scholars say to look at the results: When people start hurting others in the name of their religious truth, they've crossed the line.
2. Beware the charismatic leader.
It was one of the deadliest terrorist attacks in Japanese history. In March 1995, a religious sect called Aum Shinrikyo released a deadly nerve gas in a Tokyo subway station, leaving 12 people dead and 5,000 injured.
Two months later, Japanese police found Shoko Asahara, the sect's founder, hiding in a room filled with cash and gold bars. Kimball, who tells the story of the sect in "When Religion Becomes Evil," says Asahara had poisoned the minds of his followers years before.