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Ohio State University attacker Abdul Razak Ali Artan visited Washington, D.C. late last week and bought a knife at Home Depot, though investigators do not know if it was used in Monday's ambush, law enforcement officials told NBC News.
Artan also bought a second knife at a Walmart in Columbus the morning before he drove onto campus, rammed his car into a crowd on the sidewalk and began wildly stabbing people until he was shot dead by a police officer.
Investigators don't yet know why Artan, an Ohio State student and Somali refugee, was in the nation's capital days before the attack, which officials say may have been inspired by ISIS or the radical cleric Anwar al-Awlaki.
"It's a mystery at this point," one said. "Was he planning to do something here? Was it something else? We simply don't know."
Authorities are also trying to piece together Artan's movements the morning of the attack, asking anyone who might have seen him to contact police.
Two days into the investigation, officials said they have not nailed down a motive for the assault, which sent 11 people to the hospital.
A Facebook rant he posted just before the attack that mentions ISIS and al-Awlaki suggests either may have been an inspiration, said Angela Byers, special agent in charge for the FBI's Cincinnati division
But, she added, "it's too soon to draw any type of conclusions about whether or not this is terrorism."
The U.S. killed the American-born al-Awlaki in a drone strike in Yemen on September 30, 2011, but his influence has endured. He has been cited by more than 30 U.S. suspects linked to attacks inspired by or directed by ISIS, even though ISIS barely existed at the time of his death.
In the jumbled Facebook post, Artan called al-Awlaki a "hero" and also said the U.S. could stop "lone wolf attacks" by making peace with ISIS.
A media arm of ISIS claimed, without substantiation, that Artan was a "soldier" for the terror group. But law enforcement officials have told NBC News there is no evidence he had contact with ISIS or any other foreign organization.
"They have been known to take credit for incidents like this when the assailant is deceased and cannot refute that," Byers noted.
Investigators are still going through his social media accounts and electronic devices, trying to determine any links to extremism.
"We don't yet know if he watched propaganda videos," one official said.
Artan was born in Somalia but lived in Pakistan for seven years before coming to the United States in 2014 with his mother and six siblings. He graduated from Columbus State Community College with an associates degree in May and was a student at Ohio State at the time of the attack.
Friends in Ohio and in Pakistan have told NBC News they he appeared to be a studious, devout young man who "loved America," and they were stunned by the violence.