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.”

Robinson currently faces state charges, which were announced on Tuesday. He is being charged with aggravated murder and obstruction of justice, among others, and Utah prosecutors are seeking the death penalty in the case. Prosecutors said Robinson targeted Kirk, the co-founder of the conservative political group Turning Point USA, during the Sept. 10 event because of his “political expression.” His mother told investigators in part “that over the last year or so, Robinson had become more political and had started to lean more to the left.”

Thomas Brzozowski, who was until recently the Justice Department’s counsel for domestic terrorism, told NBC News that while Kirk’s assassination appears to meet the definition of domestic terrorism, finding a federal charge to bring against the shooter might be a challenge. There’s no federal law that makes acts of domestic terrorism a stand-alone crime, although prosecutors can seek a sentencing enhancement after conviction.

The FBI is frequently involved in domestic terrorism investigations that ultimately result in only state-level charges.

“As is always the case, the FBI needs a federal hook to initiate an investigation,” Brzozowski said. “Here, it appears that they’re acting in an assistance to state authorities’ capacity.”

Charging documents filed Tuesday also contained a series of texts between Robinson and a roommate, whom police described as “a biological male who was involved in a romantic relationship” with the suspect and transitioning to female. The roommate’s identity has not been made public.

The texts appear to link Robinson to the crime. One message alerted the roommate to a hidden note in their residence, which read: “I had the opportunity to take out Charlie Kirk and I’m going to take it,” according to the documents.

“What?????????????? You’re joking right????” the roommate apparently wrote back.

Robinson allegedly told the roommate he planned the attack for more than a week and, when asked why he killed Kirk, said: “I had enough of his hatred. Some hate can’t be negotiated out.”

Following Kirk’s shocking assassination, which has sparked a wave of grief, fear and fury on the right, Trump and his allies have threatened to come after left-wing advocacy groups that they saw as fomenting the anger that led to Kirk’s death.

Stephen Miller, the White House deputy chief of staff, said left-wing organizations amounted to a “vast domestic terror movement.”

“With God as my witness, we are going to use every resource we have at the Department of Justice, Homeland Security and throughout this government to identify, disrupt, dismantle and destroy these networks and make America safe again for the American people,” Miller said recently. “It will happen, and we will do it in Charlie’s name.”

As NBC News reported Thursday, that effort is likely to face hurdles.

“There’s not a lot of federal law on this,” Scott Walter, president of the Capital Research Center, a Washington-based nonprofit organization that tracks progressive groups and is influential in conservative circles, said then. “Frankly, the states and localities should be doing a better job [of prosecuting criminal activity], as they did in the 1960s. They have enormously more manpower.”

Since 2002, right-wing ideologies have fueled more than 70% of all extremist attacks and domestic terrorism plots in the United States, according to the Anti-Defamation League.