US Senator Ted Cruz painted over a hateful graffiti comment on a Houston freeway. The graffiti targeted conservative activist Charlie Kirk, who was shot dead during a public event in Utah last week.

In a couple of posts on his official X handle, Cruz could be seen pointing to the hateful graffiti. He could also be seen painting over the graffiti, while he captioned the video, "erasing evil".

Over the past several days, Democratic and Republican leaders have widely condemned the murder of Kirk, a 31-year-old activist and Trump world celebrity known for his hard-right views and pugnacious debating style.

A smattering of commentators, including ordinary people joking about and sometimes celebrating Kirk's death, to lawmakers and pundits dwelling on his history of bigoted rhetoric, has also surfaced, only to be targeted in organised campaigns.

At least 15 people have been fired or suspended from their jobs after discussing the killing online, according to a Reuters tally based on interviews, public statements and local press reports.

The total includes journalists, academic workers and teachers.

Others have been subjected to torrents of online abuse or seen their offices flooded with calls demanding they be fired, part of a surge in right-wing rage that has followed the killing.

Meanwhile, as part of the investigation into the case, authorities in the US arrested 18-year-old Tyler Robinson, who had scored in the top percentile on his college entrance exam and earned himself a four-year scholarship to Utah State University in Logan.

According to the investigators, Robinson had fired a rifle shot from atop a building at another university campus, killing Charlie Kirk and triggering a new round of national anxiety over rising political violence.

Investigators were still working to understand what allegedly led Robinson to that rooftop.

Officials have not yet identified a precise motive for the shooting, though they offered some clues on Friday morning in announcing his arrest.

Utah Governor Spencer Cox told reporters that a family member interviewed by law enforcement said Robinson had recently mentioned Kirk's scheduled appearance at Utah Valley University, where he was shot.

Robinson had also become more political in recent years, the family member told investigators, and authorities said he had engraved what appeared to be anti-fascist messages on bullet casings they found with the suspected murder weapon.