The idea that white supremacists could be plotting to kill a black presidential candidate will not seem farfetched to America.

The United States is home to about 900 hate organisations, the majority of them based on the notion that whites are the superior race. Collectively they form a sinister underbelly of American society, often cloaking themselves in a veil of patriotism but rejecting government authority and striving for social and political dominance over other racial groups such as African-Americans and Jews through often violent means.

FBI documents suggest that a white supremacist group assisted in the Oklahoma bombing of 1995, in which the former US Army veteran Timothy McVeigh blew up a federal building, killing 168 people. He was a member of the Aryan Republican Army, a group whose declared intention is to commit terrorist acts against the US.

According to Rick Ross, executive director of the Rick A. Ross Institute of New Jersey, which monitors cults and controversial movements, the Democratic Party’s nomination of an African-American had already prompted an upsurge in white supremacist activities. “There have been repeated rumblings against Barack Obama in the white supremacist community,” he said.

“These groups almost welcome his candidacy because it gives them a focus for recruitment, for fundraising, and they have said as much. He may end up being the most security-protected candidate of any presidential election on account of his colour.

“There are many, many groups that are deeply resentful of Obama’s candidacy, that have said hateful and horrible things about him.”

The other group mentioned in connection with the four people arrested was the Sons of Silence, one of the country’s biggest outlaw biker gangs. With its headquarters in Colorado, the Sons of Silence (SOS) are known as “one-percenters” — the term given to those that operate on the fringes of an otherwise law-abiding motorcycle community. One-percenters are dedicated to principles of personal freedom and the right to self-expression. Membership is by invitation only and their motto is Donecmors non separat — Latin that is intended to translate as “Until death separates us”.

At first sight, an alliance of white supremacists and biker gang members seems an odd one. Edward Winterhalder, former leader of a rival biker gang known as the Bandidos and now an author on gang culture, said: “Most of them are just regular guys who work during the week and have a little too much fun at the weekends. The majority of them are law-abiding, have families. They’re just regular neighbourhood guys.

“They love their Harley-Davidson motorcycles and love their brotherhood and the camaraderie of riding their bikes. They are very pro-government, they stand behind the flag. This is something they would never be involved in. Working with white supremacists would be an extremely unusual partnership.”

But Steve Cook, president of the Mid-West Outlaw Motorcycle Gang Investigations Association, said that the gangs were known to have a sub-culture — and a bad element — that could sit comfortably with racist extremism.

“I have personally seen SOS members wearing hats that say ‘Dirty White Boy’, T-shirts with swastikas and other Nazi regalia.

“I don’t believe that even a group like the SOS would knowingly, as an organisation, get themselves involved in something like this because nothing good can come of it for them.

“But would it be out of the realms for some of their associates or a member to do something like this on his own? No. Anything is in play.”

Times Recommends

• Zardari still has to tackle tough questions
• Material evidence 'debunks' Turin Shroud
• BBC criticised over Uganda gay execution poll