The federal investigation into the assassination of conservative activist Charlie Kirk has yet to find a link between the alleged shooter, 22-year-old Tyler Robinson, and left-wing groups on which President Donald Trump and his administration have pledged to crack down after the killing, three sources familiar with the probe told NBC News.

One person familiar with the federal investigation said that “thus far, there is no evidence connecting the suspect with any left-wing groups.”

“Every indication so far is that this was one guy who did one really bad thing because he found Kirk’s ideology personally offensive,” this person continued.

In addition, two of the people familiar with the probe said it may be difficult to charge Robinson at the federal level for Kirk’s killing, while the third source said there is still an expectation that some kind of federal charge is filed against Robinson.

Factors that have complicated the effort to bring charges at the federal level include that Robinson, a Utah resident, did not travel from out of state; Kirk was shot during an open campus debate at Utah Valley University. Additionally, Kirk himself is not a federal officer or elected official.

A Justice Department spokesperson said, “The investigation is ongoing and beyond that we decline to comment.”