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The value of bitcoins has dropped after the closure of the clandestine Silk Road online marketplace.

The FBI seized bitcoins worth approximately $3.6m (£2.2m) on Tuesday.

The price of a bitcoin, a virtual currency for use online, fell steeply after the arrest of suspected website administrator Ross Ulbricht.

Investor confidence may have been shaken by the association of bitcoins with illegal activity, according to a security expert.

"When there's a big bust, that's going to knock people's confidence in investing," said Rik Ferguson, a senior researcher at security company Trend Micro.

"The more a currency is associated with illegal activity, the more people will be nervous about using it," he said.

Silk Road, which allowed users to trade in illegal drugs, required transactions to be made using the virtual currency.

News of the closure was followed by a rapid drop in the price of bitcoins, according to figures from the .

The going rate for the virtual currency dropped from more than $140 (£86) to around $110, before climbing back up to $123 (£75).

Investors may have been concerned about the FBI's ability to confiscate bitcoins, said Mr Ferguson.

"Knowing that a currency could be seized or shut down could pressure people to look for alternative investment vehicles," he said.

The FBI seized the virtual currency by getting hold of encryption keys for the bitcoins, according to Jerry Brito, George Mason University's technology policy director.

The keys were made available through seized computer equipment, Mr Brito said .

The FBI then transferred the bitcoins to an address controlled by the US government, according to (PDF).