* Suspicion grows that Sweden attacker did not act alone
* Hint of collaboration comes from online activist
* Other indications include the complexity of the attack
By William Maclean, Security Correspondent
LONDON, Dec 14 (Reuters) - Information posted online about the suspect in a botched suicide attack in Sweden strongly suggests he had help from others in publicising the incident and maybe also in planning it, security experts say.
Investigators probing the first fatal bombing by a suspected Islamist militant in Europe since 2007 are examining messages posted by an admirer who has threatened more such attacks if Western troops did not withdraw from Afghanistan.
The activist, who signs messages Abu Suleiman al-Nasser, drew interest from investigators on Sunday when he published a photograph of the bomber, Taymour Abdulwahab, and named him publicly -- well before authorities or other media had done so.
"Publishing the photograph is particularly worrying," said Magnus Ranstorp, Research Director at the Center for Asymmetric Threat Studies at the Swedish National Defence College.
"Who is this person Abu Suleiman? Why does he have such good intelligence? Unless authorities can find this individual, and make tracking him down a high priority, there may be other attacks, there may be other strands to this."
"FLAME OF BATTLE"
Abdulwahab, a Swedish national of Middle Eastern origin who had studied in Britain, died in one of the explosions he is believed to have triggered in Stockholm late on Saturday night.
On Monday Al-Nasser posted another message about the incident, the first fatal bombing by a suspected Islamist militant in European capital since bombings in London in 2005.
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Reiterating a longstanding militant demand that Western forces leave Afghanistan, he noted that he had made a similar call in a Nov 19 posting about a NATO summit in Lisbon.
That earlier message was little noticed at the time as it was just one of an increasing flurry of militant threats to Europe posted online by militant propagandists over recent months.
But his access to information about Abdulwahab, suggesting he had foreknowledge of the attack, has sent investigators on a trawl through cyberspace to try to uncover more of his postings.
"If you do not (withdraw), then you should expect the flame of battle to reach the heart of Europe," he said in Monday's posting. "The battle of Stockholm is the start of a new era in our jihad, when Europe will become the arena for our battles."
Counter-terrorism specialists caution that militants have an interest in stoking fear among target populations in order to cause maximum disruption for Western socities and political stress for Western governments.
HUNT FOR CLUES IN BRITAIN
Much of the rhetoric employed by al-Nasser can be found on thousands of militant online chat rooms and websites. But his knowledge of the suspect suggests a wider conspiracy.
Sajjan Gohel of the Asia-Pacific Foundation security research consultancy in London, said the posting of the messages "clearly implies that he was part of a larger network."
But it remained to be seen whether his accomplices were a local group of "lone wolves" bonded by a common ideology or part of a wider transnational group such as al Qaeda.
Some experts say the complexity of the attack suggests it was orchestrated with others. Shortly before he was killed by the detonation of an explosives belt he was wearing, a car containing gas canisters blew up in central Stockholm.
Investigators will be looking for possible clues in Britain, where his wife and two children live and where he had studied. British media quoted neighbours as saying he was last seen in Luton two and a half weeks before the attack.
"Preparing and mounting terrorist attacks are not easy things to do," said Henry Wilkinson, a terrorism expert at Janusian risk consultancy in London.
"They take time, money and require certain skills. I would be surprised if he was able to assemble the explosive devices, no matter how crude, on his own in the short time he was in Sweden without support, especially if he hadn't had training."
A message sent by Abdulwahab in Swedish and Arabic to a Swedish news agency 10 minutes before he died, linking his action to Western military involvement in Afghanistan and cartoons deemed offensive to Islam, is also seen as a sign of possible collaboration with others. It reflects the same concerns as al-Nasser's messages.
Also, his Iraqi birth, and his mention in the mesage of visiting the Middle East to wage jihad or holy war, suggest to some that he trained in Iraq. His mention in the message of "Islamic states" is taken by some experts to refer to al Qaeda's Iraq affiliate, Islamic State of Iraq.
In 2007, that group issued a threat against Sweden, in response to a drawing published in a Swedish newspaper that depicted the head of the Prophet Muhammad on the body of a dog.
"You will learn how to kneel down in humiliation," it said. "We reserve the right to punish those who committed the crime". (Editing by Ralph Boulton)
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