An Arizona man, well-known to the FBI for suspected links to Islamic terrorists, has been identified as one of the gunmen who attacked the "Draw Mohammed" cartoon contest in Garland, Texas. 

The FBI is currently searching Elton Simpson's home, looking for clues about his accomplice.  Both attackers were killed by police after firing shots at the venue where the contest was held.

ABC News:

One of the suspects in the shooting in Garland, Texas, late Sunday has been identified as Elton Simpson, an Arizona man who was previously the subject of a terror investigation, according to a senior FBI official.

Overnight and today FBI agents and a bomb squad were at Simpson's home in an apartment complex in north Phoenix where a robot is believed to be conducting an initial search of the apartment.

Officials believe Simpson is the person who sent out several Twitter messages prior to the attack on Sunday, in the last one using the hashtag #TexasAttack about half an hour before the shooting.

Local police said two suspects opened fire late Sunday outside a community center in Garland that was hosting an event displaying cartoons of the prophet Muhammad. One unarmed security guard was injured, but other security officers managed to fatally shoot both attackers before anyone else was hurt or killed.

Followers of ISIS had been sending messages about the event in Texas for more than a week, calling for attacks. One referenced January's Charlie Hebdo massacre in France and said it was time for "brothers" in the United States to do their part.

Simpson was well known to the FBI. Five years ago he was convicted for lying to federal agents about his plans to travel to Africa, but a judge ruled the government did not adequately prove he was going to join a terror group there.

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This is another case of "Known Wolf Terrorism," as Patrick Poole of PJ Media puts it.  Few of these attacks ever come as much of a surprise to the FBI and law enforcement authorities.  Apparently, knowing who the potential terrorists are isn't enough.  They must commit an overt act – an active plan for an attack or serious threat of an attack – before they can be stopped.

This is the way of it in a constitutional republic.  It is deeply unsatisfying and frustrating.  But the same laws that shield potential terrorists shield the rest of us as well, so about all we can do is hope that authorities can interdict these attacks before they happen.  Or, as in the case of the Texas attack, kill the terrorists before they can harm civilians.