Washington (CNN) Elton Simpson, one of the two men who were foiled in an attempted attack on the Prophet Muhammad exhibit in Garland, Texas, was in private contact with known jihadists overseas who were encouraging Simpson to launch an attempted attack, multiple officials tell CNN.

Simpson and his roommate, Nadir Soofi, were killed Sunday by a guard outside the exhibit after they drove up and opened fire.

The FBI has found private communications between Simpson and some prominent terrorists whom he also had public exchanges with on Twitter including Jenaid Hussein, a British national tied to ISIS and Mohamed Abdullahi Hassan, an American now believed to be in Somalia and part of that country's al Qaeda affiliate al Shabaab, according to one law enforcement official.

Another law enforcement official described the communication with Hussein as "non public" while a third government source said the two interacted via secure communication.

U.S. investigators believe Hussein and Hassan encouraged Simpson to carry out an attack, though the sources -- who spoke to CNN on the condition of anonymity -- say that Simpson probably chose the venue and plan of attack on his own.

Hussein is described as radicalizing Simpson, pushing him to carry out an attack, officials said in describing the interaction between the two. But such influence did not mean he was giving "operational orders," such as choosing targets or helping plan the attack, but rather an effort to push Simpson to act on his own.

U.S. investigators say Simpson was being encouraged to act, but this is a murky case carried out between a self-directed lone wolf and one directed by overseas terrorists, officials explained.

Simpson had publicly connected with a Twitter account that investigators believe belongs to ISIS recruiter Jenaid Hussein. Simpson used Hussein's Twitter name in a tweet alluding to the coming Texas attack.

Several hours before the attack, Hussein tweeted an ominous message to his followers, writing: "the knifes have been sharpened, soon we will come to your streets with death and slaughter."

And Simpson himself sent a tweet with the hashtag "#texasattack" just before he launched the attack.

There were also public hints of Simpson's contacts with the American-born Hassan who, according to U.S. court documents, traveled to Somalia in 2008 from Minneapolis to join the terrorist group Al-Shabaab. A few days before the attack, Simpson tweeted a message to Mohamed Abdullahi Hassan.

"How are you doing?" Simpson wrote in a tweet, then followed with another tweet that said "dm me," meaning to send him a direct message.

Alert to the online activities, the FBI warned police in Garland about Elton Simpson's general interest in the controversial Prophet Muhammad cartoon event about three hours before Simpson and Soofi launched their attack, FBI Director James Comey said Thursday.

Comey said the FBI had seen his social media comments referencing the event but didn't know he had traveled from Phoenix to Texas, and had no indication he was planning an attack. Comey made the comments at a meeting with reporters in Washington.

One law enforcement official told CNN that the two drove all night from Phoenix to Garland ahead of the planned attack.

Law enforcement officials said the FBI had a list of suspected extremists who similarly were interested in the event and it had shared those names with local authorities. In Simpson's case, the FBI shared a photo and possible vehicle license plate. But there's no indication that information made it to officers on the street, or otherwise played a role in the quick reaction by a local officer to kill Simpson and his roommate.

Comey wouldn't say whether he believed the attack was directed or inspired by ISIS, because he said the distinction has become irrelevant the way ISIS's social media influence works.

He said there are hundreds of investigations in the U.S. of possible extremists influenced by known ISIS recruiters.

"I know there are other Elton Simpsons out there," Comey said.

Many begin with connecting with ISIS recruiters in Syria via Twitter and other public sites, then move their communications to peer-to-peer applications, which are much more difficult to track.

Comparing the FBI's task of tracking such extremists to finding a needle in a haystack, Comey said: "Increasingly the needles are invisible to us."

In Simpson's case, the FBI investigated him in 2006 for possibly trying to travel to join Somalia's Shabaab terrorist group. He was sentenced to probation, and the FBI closed its case on him in 2014, when he completed that probation.

The FBI reopened an investigation on Simpson weeks ago, in March after noticing some of his online activity that indicated an interest in ISIS, Comey said.

The FBI plans to hold a secure conference call Friday with state and local law enforcement officials, a routine event, but Comey said he and Homeland Security Secretary Jeh Johnson plan to appeal to local officials for help.

Finding potentially violent extremists before they attack is a task that requires federal and local coordination, Comey said. So he plans to tell local officials "I need your help in finding them."