London ( CNN ) -- The man in charge of security for the upcoming Olympic Games in London said Monday that preparations have been `` long and thorough , '' and expressed confidence that the event will prove to be a success . `` The national threat level at present is assessed to be substantial -- meaning that an attack is a strong possibility -- one notch lower than has been the case for much of the last 10 years , '' said Jonathan Evans , director general of the Security Service , known as MI5 , in an address at Mansion House , the official residence of the mayor of London . `` We are far from complacent . A lot of hard work still lies ahead and there is no such thing as guaranteed security . But I think that we shall see a successful and memorable Games this summer in London . '' Is this the world 's greatest athlete ? Evans said members of his service had been involved in advising on the design and security of the venues , in the accreditation of those working there and in intelligence collection and analysis . `` The preparations have gone well , '' he said . `` No doubt some terrorist networks have thought about whether they could pull off an attack , '' he acknowledged . `` But the games are not an easy target and the fact that we have disrupted multiple terrorist plots here and abroad in recent years demonstrates that the UK as a whole is not an easy target for terrorism . '' The Royal United Services Institute , a defense and security think tank , has identified 43 potential plots or serious incidents in Britain since the terrorist attacks in the United States on September 11 , 2001 , he said . Saudi Arabia allows women to compete . Evans credited an investment in counterterrorism with having succeeded since then in `` identifying , disrupting and suppressing terrorism before it succeeds . You could say that we are near to reaching a form of stalemate -- they have n't stopped trying but we have got better at stopping them . '' He added that he was seeing a change in international threats , with incidents involving Pakistan and/or Afghanistan falling from about three-quarters of all such threats to fewer than half . `` We appear to be moving from a period of a deep and focused threat to one where the threat is less monolithic but wider . Al-Qaeda affiliates in Yemen , Somalia and the Sahel ( region of Africa ) have become more dangerous as al Qaeda in Pakistan has declined and we see increasing levels of cooperation between al Qaeda groups in various parts of the world . '' In addition , `` a small number of British would-be jihadis '' have been traveling to Arab countries to find training and opportunities for militant activity , he said . `` Some will return to the UK and pose a threat here . This is a new and worrying development and could get worse as events unfold . So we will have to manage the short-term risks if there is to be a longer-term reward from the Arab Spring . '' Photo finish in 100-meter trial . The Security Service , or MI5 is the British domestic spying agency -- sister of MI6 foreign intelligence service , where fictional spy James Bond worked . Both services emerged from World War II , but neither were acknowledged to officially exist by the British government until 1992 . Evans also gave a blunt warning about the dangers from Iran , saying `` a return to state-sponsored terrorism by Iran or its associates , such as Hezbollah , can not be ruled out as pressure on the Iranian leadership increases . '' CNN 's Dan Rivers contributed to this report .
Preparations have been `` long and thorough , '' says Jonathan Evans . `` The games are not an easy target , '' he says . Concerns are moving to a less monolithic but wider threat . British would-be jihadis are `` a new and worrying development ''