Before Texas Shooting, Suspect Traded Tweets With ISIS Propagandist

Elton Simpson communicated with American fugitive.
2:27 | 05/05/15

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Transcript for Before Texas Shooting, Suspect Traded Tweets With ISIS Propagandist
And we begin tonight with an ABC news investigation. Isis is now claiming its first victory here on American soil, claiming responsibility for that terror in Texas. The gunmen who opened fire, the traffic cop shooting them both dead. We all saw the chaos that followed. Isis now taking credit for what those men did. But offering no proof tonight. Also this evening, word of another person from the united States involved before this horrific scene played out. ABC news has learned about secret conversations and messages that allegedly led up to the deadly encounter in Texas. The burned-out car when it was all over. Tonight, the hunt for the third person, an American fugitive. And ABC's chief investigative correspondent Brian Ross with those messages, right here tonight. Reporter: Isis today claimed responsibility for these scenes of America under attack -- bullets flying and police scrambling. The terror group said the two heavily armed men killed in the attack, Elton Simpson and nadir soofi, were what it called Isis soldiers. Isis offered no proof it had planned or directed the assault, but today there was new evidence of the critical role of a mysterious Isis recruiter, the man posing here with a snake, an American from Minneapolis known online only as miski. Miski's been pushing for violence for many months, as long as we've been following him, he's been pushing for individuals to commit acts of violence. Reporter: Authorities say miski's real name is Mohamed hassan, a fugitive wanted by the FBI since 2009, after he left the twin cities as a teenager to join terror groups in Africa. Messages obtained by ABC news between miski and the Texas ringleader Simpson show miski pointing to the Texas cartoon contest and calling for an attack. "It's time for brothers in the U.S. To do their part," he wrote. Two days later, miski writes to Simpson, "One individual is able to put a whole nation onto its knees." He's very, very effective. I'd call him a recruiter, I'd call him a propagandist, I'd call him an inciter. Reporter: The day before the attack, miski posted this picture with the message "May Allah protect us from the snakes of the hereafter." Simpson tweeted back, "Amen, bro. Amen." Brian with us now. And Brian, the few has been tracking these men for years? Reporter: That's absolutely right. For the last three, four months, they have determined that he's radicalized or recruited at least 16 Americans and a dozen other westerners to become part of Isis. The hunt for that American fugitive tonight. Brian, thank you.

This transcript has been automatically generated and may not be 100% accurate.

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