The suspects, aged 52 and 21, were arrested on suspicion of explosives offences on Friday morning at separate addresses in Caernarfon, North Wales, by officers from the Metropolitan Police and local police. Police were granted more time to question the men on Saturday afternoon. Searches were being carried out at two addresses in Caernarfon in connection with the arrests. Police were called at about 12:40pm on Wednesday after a suspicious package was found at the MI6 building in Vauxhall, south London. A second suspected parcel bomb was discovered at a sorting office in south London at around 4:30am on Thursday. A spokesman for the Metropolitan Police said: "The Metropolitan Police Service is investigating two suspect packages addressed to premises in central London. "Both packages have been recovered by police." The force said the packages had the potential to cause injury, but no-one was hurt. MI6's fortress-like headquarters is a London landmark. In 2000, it was slightly damaged in a rocket attack by Irish Republican Army dissidents No expense was spared when the huge honey-coloured building was built on the banks of the Thames to house Britain's overseas intelligence network. The headquarters of Military Intelligence Section 6, completed in 1992, cost 240 million to build and furnish and is rumoured to include bunkers capable of withstanding a nuclear strike. Known as Russia House, the building is one of the most secure in the world and lies on the south bank of the Thames at Vauxhall Cross. Diagonally opposite is Thames House, which is home to MI5 - Britain's domestic intelligence network. The MI6 building was designed by award-winning architect Sir Terry Farrell, who has worked on high-profile building schemes and city masterplans around the world. He is also the man responsible for creating the Edinburgh International Conference Centre and the National Gallery of Scottish Art and History in Glasgow. Margaret Thatcher originally approved 252 million for the projects to provide homes for Britain's two intelligence services. But costs soon spiralled as both buildings were fitted out against possible terrorist attacks and the final bill was 547m. The end result was two new landmarks which remain a mystery to all except the privileged few allowed inside their doors. Speculation has been rife as to exactly what security measures MI6 has taken to keep its secrets hidden. The most sensitive areas are below street level to protect them from terrorist attack.