A Californian teenager is in custody on suspicion of terrorist threats aimed at the US Open of Surfing where New Zealand's four top surfers are competing.
Huntington Beach police arrested the 16-year-old male after New York police became concerned at "dark and disturbing" postings on social media that included images of him holding guns.
They had initially believed the postings were directed at the looming US Open tennis championship in New York, but alerted their Californian counterparts when realising they were centred on the surfing championship at the famous beach.
New Zealand has Paige Hareb, world junior champion Ella Williams and top men Billy Stairmand and Ricardo Christie competing at the US Open.
Having established the threats as "credible", police monitored the suspect and arrested him on terrorist threat charges at the weekend, seizing a handgun and shotgun at his residence, according to American media reports.
Both firearms were legally acquired by the suspect's family members. Computers and electronic devices were also seized and the youth remains in custody.
Local police chief Robert Handy made it clear to the Huntington Beach Post that they weren't prepared to take risks, especially after the recent fatal university campus shootings in California that had been signalled by the gunman on social media.
"If you look back at lessons that we've learned in law enforcement, like the Isla Vista shooting and other [events], that some of the things that had been posted earlier and all of that, law enforcement pays a lot of attention to those things," Handy said.
"We take them very seriously, so we acted very quickly on trying to determine what this guy's capabilities was, who he was and where he was at."
Handy said the teen had indicated he was "just messing around", making the posts under a pseudonym. He had no prior criminal records and it was unknown if he had any previous mental health issues.
The surfing contest, that attracts massive crowds, has had a history of social problems including the famous 1986 riots that saw police cars burned.
Rioting at last year's event has led to several safety measures being introduced.
Organisers have dropped the live music stage and public skate bowl as well as outlawing alcohol sales and consumption this year.
But that didn't prevent police making 78 arrests as competition got under way with surfing trials events last weekend. Police said most of the arrests were for drunkenness and disorderly conduct in the downtown area.
Williams started her junior event today. She won her opening heat but was eliminated in the quarter-finals.
The other New Zealanders are waiting to start their competitions. Hareb's event at the US Open is a full world tour contest while Stairmand and Christie are involved in a prime-rated world qualifying competition.
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