Did ISIS instruct Texas gunmen to attack?

Investigators are trying to determine whether Elton Simpson and Nadir Soofi, the two gunman killed at the Texas art contest Sunday night, received specific directions from the Islamic State of Iraq and Syria (ISIS) or were simply inspired by the terrorist organization to carry out attacks in the U.S., reports CBS News correspondent Jeff Pegues.

In the last month, something sparked law enforcement's interest in Simpson. It's not clear exactly what that was, but the Arizona man had multiple social media accounts and tweeted out hints of the attack before it was set in motion.

One anti-terror watch group says the evidence is online in Twitter exchanges between Simpson and a man named Mujahid Miski, whose real name is Muhammed Abdullahi Hassan.

"He propagandizes, he incites, he recruits via social media, particularly in his case, Twitter," Extremism Project executive director David Ibsen said.

Just last month Miski tweeted: "The brothers from the Charlie Hebdo attack did their part. It's time for brothers in the #US to do their part." Simpson tweeted back: "When will they ever learn? They are planning on selecting the best picture drawn of Rasulullah (Prophet Muhammad) in Texas."

"I'd characterize that conversation as being evidence of incitement and propagandizing on behalf of extremist groups, targeted toward individuals located in the U.S., compelling them and encouraging them to commit acts of violence in the U.S. against Americans," Ibsen said.

Before Sunday's attack, Simpson pledged allegiance to ISIS and wrote: "May Allah accept us as Mujahideen."

Then Simpson, along with his roommate Soofi, opened fire on police in Garland, Texas. They were well-armed with two AK-47 assault rifles and two handguns. A police officer shot and killed them both.

Simpson was the subject of a terrorism investigation in 2006 and was convicted in 2011 of lying to the FBI about wanting to join terrorists in Somalia. He received three years' probation.

"I wonder if he just snapped," attorney Kristina Sitton, who represented Simpson in that case, said. "It never occurred to me that he would actually follow through on this."

A law enforcement official told "CBS This Morning" that Simpson and Soofi were not considered "high priority threats." They are like several thousand individuals U.S. law enforcement is following across all 50 states.