LONDON (Reuters) - Last December, a purported Islamic fighter called Amir Ghrozni posted a cryptic message on a password-protected website connected to al Qaeda.
"London, perhaps the punishment is near," the message said. "Await the glad tidings, as we will never calm down."
Similar threats to the 2012 Olympic Games, which begin in London on July 27, have surfaced since.
A February message on a different website urged readers to "be the initiator O Soldiers of Usama, as London has been beautified with its nicest decorations and the best of dresses."
In April, someone calling themselves "Kafir bel-Taghoot" reposted a 2003 religious tract by a militant Saudi Imam offering Islamic arguments legitimizing the use of weapons of mass destruction.
"Due to the nearing of the London Olympics...I gift you the holistic book that displeases the advocates of surrender and defeater of the advocates of reconciliation and tolerance," the message said.
Official and private experts say such messages are representative of "chatter" that U.S. and European intelligence agencies are picking up about the London Games.
But they point out that the threats are utterly non-specific - no dates, targets or methods of attack are mentioned.
The absence of intelligence pointing to specific plots or attacks during the two-week Olympic period was confirmed by officials from multiple intelligence and security agencies in Europe and the United States.
It means "lone wolf" attacks which give them little advance intelligence are their main concern.
In a June speech, Jonathan Evans, head of Britain's principal counter-terrorism agency, the Security Service, also known as MI5, reported that the British government's official assessment of the threat to the games was "Substantial" - meaning that "an attack is a strong possibility."
But he also acknowledged that this assessment is "one notch lower than has been the case for much of the last ten years", when British authorities rated the threat to London as "Severe".
A U.S. national security official said earlier this week there was "irony" in the fact that British authorities are making and publicizing massive security measures which are considerably more elaborate and extensive than known threats.
A report published last week by a parliamentary committee which supervises Britain's intelligence services said it was told by MI5 that there were three principal sources of potential threats to the London games: an attack by al-Qaeda and its affiliates on the Games or participants, such as Americans or Israeli athletes; an attack or disruptive hoax by renegade Irish Republicans or "clashes between rival groups or ethnicities that would be present in London during the Games."
Not included on this list was a category of threat which British and U.S. officials say is what they worry about most with respect to the Olympics: the possibility of an attack by a "lone wolf" individual or small cell of militants who radicalized themselves, often via the internet, and are unknown to intelligence agencies or police.
U.S. and European officials said the Olympic threats posted on al Qaeda websites amount to little more than exhortations to self-radicalized militants to target the Olympics with whatever means they have at their disposal.
This is consistent with agencies' assessment that the most worrying attack threats facing the Olympics are posed by people who are either completely unknown to intelligence agencies or are, like British militants who attacked London's transport system on July 7, 2005, known to authorities but not regarded as high-priority dangers.
At least in part to militate against such threats, and to deter potential "lone wolves", British authorities have recently conducted a series of roundups and arrests of suspected Islamic militants.
UK and U.S. sources familiar with the investigations say none of them related to threats focused specifically on the Olympics. In one case, two individuals picked up near the site of an Olympic boating venue were subsequently released without charge.
In another case, a group of suspected militants recently rounded up north of London are believed to have been plotting a violent confrontation with an anti-Islamic right-wing group.
In a third case, a militant subject to a British counter-terrorism order was picked up after he took train journeys, in violation of government instructions, near the main Olympic venues. Officials now acknowledge that the man may have been travelling to visit his lawyer.
Other investigations also have resulted in raids or arrests near Olympic sites. But a British official said East London neighborhoods surrounding the main Olympic sites are a frequent venue for investigations directed against Islamic militants, and the timing of recent police operations in the area is largely coincidental with the Olympics.
British authorities remain concerned about the possible influence on home grown radicals of Anwar Awlaki, and American-born militant who was arguably the most influential English-language internet apostle of violent Jihad until he was killed in a CIA drone-strike last September at a hideout in Yemen.
In the last two years, UK authorities have brought charges against at least three individuals who allegedly became involved in plotting or carrying out violence after following Awlaki on the internet or reading Inspire, an English-language glossy magazine posted on the internet by Al Qaeda in the Arabian Peninsula (AQAP) the al Qaeda affiliate with which Awlaki was associated.
Inspire Magazine also figured in one of the most recent UK counter-terrorism roundups.
British national security officials and their American and European counter-parts have largely discounted the threat from the central al-Qaeda organization founded by Osama bin Laden and led today by his long-time Egyptian deputy, Ayman al Zawahiri.
Al Qaeda's core group, officials assess, has been too badly damaged by U.S. drone strikes and other counter-terrorism measures to be capable of launching an elaborate multi-faceted attack on London similar to the September 11, 2001 attacks on New York and Washington D.C.
Lower level attacks, which could range from militants armed by Yemen-based extremists with exotic weapons like underwear bombs to "Mumbai-style" shootings by men armed with guns, are what worry Olympics security personnel more deeply.
Editing by Philippa Fletcher