Investigators say Robinson was steeped in online culture, pointing to inscriptions on shell casings linked to the case.

Two casings displayed apparent references to online trolling humour. The inscription"notices bulges OwO what's this?" on a fired shell casing may refer to a "copypasta" - a piece of text that is repeated over and over again, often to troll people online. Another casing, that was unfired, was inscribed with the words"If you read this, you are gay lmao" - again an apparent reference to a trolling joke.

Meanwhile, other casings could be interpreted as sympathetic to Antifa, or the anti-fascist movement, a loose collection of far-left activists who have been active in the US over the last decade and often demonstrate against Trump policies and far-right groups.

One unfired casing had the words"Hey fascist! Catch!" and an up, right and three down arrows.

The three down arrows alone could be a common symbol used for anti-fascism. As a whole, the arrows could reference a sequence of control inputs in a video game - although this remains unclear, and authorities have not yet released images of the casings.

A second casing was inscribed with lyrics to the song"Bella Ciao" that honours World War Two-era partisans of the Italian resistance who fought Nazi Germany.

Authorities said Robinson also appeared to have been active on Discord, the social media platform primarily used by gamers, but now also popular with other communities.

His roommate reportedly showed investigators a series of messages on Discord from a contact named"Tyler" referencing a rifle "drop point" and instructions about retrieving and hiding the weapon.

Discord later issued a statement saying his account had been suspended."We have removed the suspect's account for violating our off-platform behavior policy," it said.

For now, Robinson remains in custody as prosecutors prepare formal charges.

The investigation into his background, motives, and potential affiliations continues in what officials are calling one of the most significant political killings in recent US history.