BANGKOK, Nov 5 — The arrests of three people accused of spreading rumours about the health of Thailand’s revered King, sparking a stock market plunge as a result, has raised eyebrows and sent jitters through the media.

The trio were arrested under Thailand’s draconian Computer Crimes Act, and more arrests are expected.

A statement on Monday by Tharit Pengdit, director-general of Thailand’s Department of Special Investigation (DSI), suggested that the net could be cast wider in a probe of what he called a coordinated effort — involving “persons, juristic persons, local news agencies and foreign news agencies” — to attack the stock market.

Thai stocks plunged 7.2 per cent on Oct 14 and 15 as rumours circulated over the health of King Bhumibol Adulyadej, who had been in hospital since Sept 19.

Daily statements had reported that his health was improving, but Thais are jittery about his frail health and advancing age. He celebrates his 82nd birthday next month. The King is Thailand’s ultimate moral authority, and is seen as the last resort at times of severe national crises.

In a report yesterday in the Post Today, a Thai-language business newspaper, the DSI said it was looking at charging a news agency in Hong Kong over the reporting of the stock market fall — an obvious reference to Bloomberg Business News. Its bureau in Hong Kong was the first to report the stock market sell-off on Oct 14.

The report came out at 3.38pm Thai time after the local bourse had already fallen for several hours.

On Sunday, Thai nationals Thiranan Vipuchanan and Katha Pajajiriyapong, both former brokers, were arrested under the Act on suspicion of passing on false information that could “threaten national security or cause public panic”.

Thiranan’s arrest, however, has left some critics baffled because she filed a posting on website Prachatai.com after the first Bloomberg report had appeared — and after the market had closed. The post was a translation of the Bloomberg report.

Prachatai.com is an independent site but, under Thailand’s current polarised political climate, is considered pro-Thaksin Shinawatra, the ousted former prime minister, among the ruling elite. Chiranuch Premchaiyaporn, director and Web moderator of Prachatai.com, herself has been charged under the Act, which came into force in June 2007.

Katha posted his comment on the website of Fah Diew Kan, a leftist journal. The daily Thai Rath yesterday quoted Minister for Information Technology Ranongrak Suwanchawee as saying that Katha posted “inauspicious” content on several websites and forwarded it to friends.

At least two of them forwarded the message to others. Those two would also be charged, the minister said, adding that the Internet service providers (ISPs) of Prachatai.com and the Fah Diew Kan website could be shut down if they continued to host sites that threatened national security.

A third person accused of spreading rumours about the health of the King was arrested on Tuesday. Somjet Sitthiworakul is reportedly a small-time snooker hall owner.

All three have been granted bail.

In an editorial on Tuesday, the Bangkok Post said the arrests were troubling.

“The Computer Crimes Act, denounced when it was passed under the military regime, has turned out much like its critics feared — being used as a catch-all law to stifle criticism and to intimidate the media,” it said.

“The Bloomberg report did not speculate about the King’s health. It reported the fact, as did hundreds of news outlets, that a sell-off at the SET (Stock Exchange of Thailand) was the result of ill-intentioned rumours.”

The daily added: “A vital and urgent question is whether Thiranan and Katha are scapegoats. On the evidence released by police, the two neither started nor profited from the rumour.”

Supinya Klangnarong, a board member of the Thai Netizens Network, told The Straits Times: “The Act’s intent was to combat online crime (but) it is being used for political purposes.”