Fort Lauderdale Suspect's Motive Remains a Mystery

Federal authorities are trying to piece together just what led the suspect, a 26-year-old Army veteran, to go on a murderous rampage.
1:49 | 01/07/17

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Transcript for Fort Lauderdale Suspect's Motive Remains a Mystery
And you heard senator Nelson tell me earlier the suspect was found with a military I.D. On him. It was not long after, a clearer portrait began to emerge of the suspect. And it turns out tonight, he was known to the FBI. Here's our chief investigative correspondent Brian Ross. Reporter: Esteban Santiago remains a mystery tonight to federal authorities, still not sure what led this 26-year-old army veteran to allegedly go on his murderous rampage. But some of his social media accounts are now raising concerns. He was born in New Jersey but grew up in Puerto Rico. Santiago's Instagram profile page has photos of him in uniform as a member of the army National Guard in Puerto Rico and Alaska. He was deployed to Iraq for a year and honorably discharged four months ago. Santiago's profile statement reads, "Here's to finding whether or not your mission on Earth is finished. If you're still alive, it isn't." Federal authorities tonight tell ABC news that Santiago recently received mental health treatment, after he showed up at the Anchorage, Alaska, FBI office, claiming the government was forcing him to watch ISIS videos. Agents described him as incoherent. Tonight, FBI agents and police are at Santiago's last known address at this trailer park in Anchorage. And outside the home of his relatives in union city, new Jersey. Trying to decipher the troubled life of Esteban Santiago, and what brought him to the airport in ft. Lauderdale with a gun. Brian Ross with us here tonight on this fast-moving investigation. And federal officials looking at this as a possible terror investigation? Reporter: That hasn't yet been decided. His rants about ISIS back in Anchorage, it seems increasingly likely the joint terror task force will take over this investigation at some point soon. So, this could take a turn. Brian Ross tonight, thank you.

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

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