US cyber security firm Cloudflare will terminate online message board 8chan as a customer after a shooter used the forum just before killing 20 people in El Paso, Texas.

The ban will leave the controversial website severely vulnerable to a cyber attack.

The El Paso shooter is believed to have posted a four-page statement on 8chan, and called Saturday’s Walmart attack “a response to the Hispanic invasion of Texas”.

The suspected Walmart shooter has been named as 21-year-old Patrick Crusius, from Allen, Texas. Allen is about 1046 kilometres east of El Paso, which lies along the Rio Grande, near the US-Mexico border.

“We just sent a notice that we are terminating 8chan as a customer effective at midnight tonight Pacific Time,” Cloudflare chief executive Matthew Prince said in a blog on Monday.

“Based on evidence we’ve seen, it appears that he (the gunman) posted a screed to the site immediately before beginning his terrifying attack on the El Paso Walmart, killing 20 people.”

Mr Prince said such messages of support for hate-motivated killers were not isolated on 8chan.

“Nearly the same thing happened on 8chan before the terror attack in Christchurch, New Zealand,” he said.

We just sent notice we are terminating service for 8chan. There comes a time when enough is enough. But this isn't the end. We need to have a broader conversation about addressing the root causes of hate online. https://t.co/ZsctDpswM5

— Matthew Prince 🌥 (@eastdakota) August 5, 2019

The El Paso shooter specifically referenced the Christchurch shootings and appears to have been inspired by the largely unmoderated discussions on 8chan, which glorified the previous massacre. An Australian-born man faces 89 charges, including New Zealand’s first terror charge, in relation to the mosque attacks.

“In a separate tragedy, the suspected killer in the Poway, California synagogue shooting also posted a hate-filled “open letter” on 8chan,” Mr Prince said.

“8chan has repeatedly proven itself to be a cesspool of hate.”

He said 8chan had shown it was lawless – and its lawlessnes had caused multiple tragic deaths.

“We reluctantly tolerate content that we find reprehensible, but we draw the line at platforms that have demonstrated they directly inspire tragic events and are lawless by design,” he said.

“8chan has crossed that line.”

Australian telcos, including Telstra, Optus and Vodafone, banned 8chan after the Christchurch shooting, following a plea from the New Zealand government.

-with AAP