The National Crime Agency in the United Kingdom arrested Tuesday four men suspected of selling illegal drugs on Silk Road, which U.S. law enforcement shut down last week with the capture of the alleged owner and operator, Ross William Ulbricht, in San Francisco. [Silk Road bust demonstrates feds penetration of Deepnet]The latest arrests included three men in their early 20s and one in his 50s, all suspected of being sellers on Silk Road, which was one of the world's largest online marketplaces for illegal drugs, the BBC reported. The NCA said more arrests were expected in the coming weeks. In the U.S., federal authorities arrested on drug charges last week Steven Lloyd Sadler, 40, Bellevue, Wash., who was allegedly one of the "top 1 percent of sellers" on Silk Road, the KrebsonSecurity blog reported. Also arrested was Sadler's roommate, Jenna White. The U.S. Federal Bureau of Investigation and the NCA have made it clear that they are as focused on finding sellers on illicit marketplaces as they are in hunting down the operators. While Silk Road was best known as a place for illicit drugs, other marketplaces specialize in other illegal activities, such as child pornography and the trading of stolen credit cards. "These arrests send a clear message to criminals; the hidden Internet isn't hidden and your anonymous activity isn't anonymous," Keith Bristow, director general of the NCA, told the BBC. "We know where you are, what you are doing and we will catch you." Bristow went on to say that it was impossible for criminals to erase their digital footprint. "No matter how technology-savvy the offender, they will always make mistakes," he said. Bristow's comments are not just bravado, security experts say. "The arrests certainly demonstrate that the long arm of the law does indeed extend into the dark Web," Raj Samani, vice president and chief technology officer for McAfee in Europe, said. The "dark Web" refers to the Tor anonymity network used by illicit markets like Silk Road. Tor, which is often used by political activists to avoid government surveillance, directs traffic through a volunteer network of more than 3,000 relays that make it extremely difficult to trace Internet activity. Despite the anonymity, the latest arrests show law enforcement is finding ways around the obstacles set up by criminals. "Many people are spending a great deal of time observing the dark Web through a variety of means," Will Gragido, senior manager of Threat Research Intelligence at RSA, said. [Blackhole exploit kit creator arrested in Russia]"Law enforcement agencies are taking advantage of this work that is being largely done by private research organizations." In the Silk Road case, FBI agents posed as buyers to make purchases and to gather information on sellers. Federal authorities say Ulbricht's downfall stemmed from a number of alleged mistakes, such as using his Gmail address in promoting Silk Road, managing the site from an insecure network and failing to use encryption for all communications. "Even with the use of technology purported to maintain anonymity, simple mistakes can lead to arrest," Samani said.