Investigators have been trying to establish a motive for the killing.
Cox has previously said that Mr Robinson, a Utah native, was “deeply indoctrinated with leftist ideology”. Public records reviewed by the BBC suggest Mr Robinson had in the past registered as an unaffiliated, or nonpartisan, voter in Utah. His parents, meanwhile, are registered Republicans, according to state records.
However, according to Cox, a family member told investigators that Mr Robinson had become “more political” in recent years and had specifically mentioned Kirk’s impending visit to Utah Valley University during a dinner.
One of Donald Trump’s top officials, deputy chief of staff Stephen Miller, has alleged there was an organised campaign behind the assassination but no evidence has been presented so far that suggests Mr Robinson was working as part a group.
He was speaking on Kirk’s podcast, the Charlie Kirk Show, which was guest-hosted by his friend, Vice President JD Vance, in an episode that aired on Monday.
Investigators are also looking at social media accounts used by Mr Robinson, according to CBS, and any signs that anyone may have known about or encouraged the shooting beforehand.