The key component to any terrorist attack is luck. There's an enormous amount that needs to go right at key moments — but terrorists also need luck in escaping mistakes they've made. It's that kind of luck that's providing early if uneasy answers in the case of the alleged Boston Marathon bombers, which became official as the U.S. charged Dzhokhar Tsarnaev in his hospital bed Monday.

The FBI never flagged Tamerlan Tsarnaev.

As has been widely reported, the FBI interviewed the elder brother, Tamerlan, in 2011. The Times reports that the request came from the Russian government.

The Russians feared Tamerlan could be a risk, and said their request was “based on information that he was a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared to leave the United States for travel to the country’s region to join unspecified underground groups,” the F.B.I. said in a statement Friday.

In a look at Tamerlan's trip to Chechnya and Dagestan last year, the paper notes that the agency "interviewed Mr. Tsarnaev and his family in Boston but found no sign of terrorism activity at that time."

When Tamerlan returned from Asia, he applied for citizenship. The Department of Homeland Security, as part of its research into whether or not he should be granted that status, contacted the FBI to learn more about the 2011 interview.

The F.B.I. reported its conclusion that he did not present a threat.

At that point, Homeland Security officials did not move to approve the application nor did they deny it, but they left it open for “additional review.”

The FBI may have missed another clue. During an appearance on Fox News this morning, Senator Lindsey Graham described a conversation he had with an assistant director of the agency. The FBI may not have known Tamerlan went back to Asia last year, visiting two regions of Russia tied to terrorism, because "apparently when he got on the airplane, they misspelled his name, so it never went into the system that he actually went to Russia."

One component of the Tsarnaevs' alleged success had nothing to do with luck. When the two brothers immigrated to the United States, seeking asylum with their father, they were 15 and 8 years old. There was apparently nothing in the family's background that would suggest the sons' alleged future crimes. Only after Russia expressed concern about Tamerlan did the FBI have any interest in him; it was that interview that played the biggest role in denying him citizenship — not anything that could have been detected in 2002.

Members of the local Muslim community were alarmed by Tamerlan.

In January, Tamerlan was asked to leave a Muslim prayer service at a local mosque. According to Boston.com:

Tamerlan Tsarnaev, one of the brothers accused of bombing the Boston Marathon, angrily disrupted a January talk at a Cambridge mosque when a speaker compared the Prophet Mohammed and the peace activist Martin Luther King Jr., the second time in recent months that Tsarnaev’s radical theology collided with mainstream Muslim faith at a public religious talk.

Worshippers "shouted him out of the mosque" after his outburst. The second instance occurred last November, with Tamerlan challenging a speaker on whether or not celebrating secular holidays was appropriate.

A trustee of the mosque downplayed the incidents. But at least one member of the congregation told the Los Angeles Times about his concerns about Tamerlan, who he called "not really so nice."

The Tsarnaev brothers somehow got firearms illegally.

One of the four people who allegedly died at the Tsarnaevs' hands was an MIT police officer, Sean Collier. Unlike the victims at the marathon, Collier was shot to death on the school's campus.

According to the Cambridge, Massachusetts, police department, neither brother had a license to own a firearm. According to reports, police recovered two handguns from the car the brothers stole. When Dzhokhar was captured Friday night, he had with him an M-4 carbine rifle.

As the Huffington Post notes, it's possible neither could have owned a handgun legally.

"The younger brother could not have applied as he is not 21 years of age and the older brother did not have a license to carry and we have no record of him ever applying," [police spokesman Dan] Riviello said. …

Department of Homeland Security officials decided not to grant him citizenship after what The New York Times called a "routine background check" revealed that FBI officials had interviewed him in 2011, at the request of the Russian government, which was concerned that he had ties to Chechen terrorists. He was also reportedly involved in an episode of domestic violence in 2009 against his girlfriend.

One of the questions on Massachusetts firearm license application deals with past domestic violence charges.

That the brothers nonetheless ended up with weapons is its own stroke of luck. How they ended up with the weapons will certainly be investigated by the federal officials. Regardless, the failure of authorities to determine that Tamerlan in particular was in illegal possession of weapons is a not-uncommon occurrence — one that, if detected, could have halted the attacks.

There may have been other missed signals.

It's very easy, after a crime such as the one that occurred last week, to see significance in unrelated events. At BuzzFeed, former friends of Tamerlan's see new significance in his reaction to the unsolved fall 2011 murder of a close friend. While there doesn't appear to be any evidence that Tamerlan might have been involved, it's hard not to wonder what a more successful investigation would have turned up — especially given that it was in the same time period that the FBI was investigating his background.

After the bombing, new attention was also paid to local news reports of several explosions in Hanover, a town about 45 minutes southeast of Boston. There's no indication that the Tsarnaevs were involved in the blasts, which involved "homemade explosives." The State Police were called in to detonate other unexploded devices found at the scene. While highly circumstantial, it seems likely that someone seeking to use an explosive as a terror device would want to test how effectively it worked. Whoever set off the devices managed to avoid being captured by the police. If by chance it was the Tsarnaevs, that could mean another missed opportunity.

There's one piece of luck that wasn't on the bombers' side. According to medical authorities, all of those injured in last Monday's attack are expected to survive.