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The Big Question: Why has MI5's new chief gone public, and can we trust what he says?

Why are we asking this now?

The head of MI5, Jonathan Evans, was on display yesterday after he had invited selected journalists to visit him in his office in Millbank, overlooking the Thames, so that he could make a few uncontroversial comments to mark the centenary of the spy network. It was the first time in MI5's 100-year history that a serving head of MI5 had met the press – and even on this occasion, Mr Evans was cautious about which journalists were invited. He gave nothing away.

What message was the MI5 boss trying to get across?

Others may be losing their jobs through recession, but MI5 is still recruiting. Its payroll will rise to 4,100 by 2011, compared with 2,800 two years ago, and fewer than 2,000 before September 2001. Mr Evans emerged from the shadows to emphasise the importance of having a large well-funded organisation to combat the threat of domestic terrorism. He hinted, without expanding on the point, that the problem could increase as the economic situation gets worse. In short, he was telling the public, and indirectly the government, that MI5 is well worth its expanding cost.

The photograph that went with the interview showed him without a jacket or tie, with his top shirt button undone and his sleeves rolled up, to show that this is a man in tune with the times, and not some old-fashioned Sir Humphrey, though the effect was spoilt somewhat by the revelation that our chief of domestic spooks is bald.

Is Mr Evans a typical MI5 boss?

For 15 years before Evans was appointed, in 2007, MI5 was run by women Director Generals. Further back, they were all men, but they did not show their faces in public. The appointment of Stella Rimington in 1992 was the first to be announced publicly. Her successor was Eliza Manningham-Buller. Naming the new boss was part of a gradual process of lifting some of the secrecy around MI5. In 1998, MI5 launched its own website, and an official history is in preparation.

MI5 is an equal opportunities employer, then?

Mr Evans disclosed that 47 per cent of MI5 staff are women, eight per cent are from ethnic minorities, and the average age is under 40. Rather a large proportion of the women staff are believed to be old girls of Cheltenham Ladies College, whose buildings are within spitting distance of the vast listening station run by GCHQ, which is the other major branch of the intelligence services, besides MI5 and MI6.

What's the difference between MI5 and MI6?

MI5 deals with internal threats to Britain's security, MI6 other handles operations abroad. So those fictional heroes, James Bond and George Smiley, worked for MI6; so did the real-life traitors, such as Kim Philby. Famous MI5 officers are rare, either in real life or fiction, but in the 1980s an ex-MI5 officer named Peter Wright briefly became a household name when he published his memoirs. Normally, that would have been impossible, because of the Official Secrets Act, but he brought them out in Australia, embroiling the British government in a court case on the far side of the world, which it lost.

Have any other MI5 officers emerged from secrecy?

Before the Wright case, there was the Michael Bettaney scandal. He was a self-destructive drunkard who tried to become a Soviet spy, but his behaviour was so suspicious that the chief KGB officer in the London embassy did not trust him. He was arrested and sentenced in 1984 to a long prison term before he could pass on any secrets, but there were awkward questions about how why MI5 had recruited someone so erratic and employed him for eight years without spotting his strange behaviour.

Procedures were supposedly tightened up after his conviction, but in 1997, another officer, David Shayler, came to public notice when he was caught passing information to journalists. Shayler now says that he is the Messiah, thereby fortifying the suspicion that some very odd people have worked for MI5.

Was there ever an agent called Carruthers at MI5?

Since the full list of past and present MI5 operatives remains secret, we don't know if the organisation has ever employed anyone named Carruthers. But a spy by that name played a part in its launch, though he did not exist in reality. He was a character in the 1903 novel Riddle of the Sands, by the Irish nationalist Erskine Childers, the first 20th-century spy novel.

The story, which was about a German plot to invade Britain, was realistic enough for the first lord of the Admiralty, Lord Selborne. He set up the Secret Service bureau in 1909 to hunt for German spies. By the time war broke out, five years later, the bureau employed 14 officers and could claim credit for uncovering at least one German spy ring who were sending message to Berlin in invisible ink.

Who is watched by MI5?

After 1945, the organisation stopped looking for Nazi agents and turned its gaze on the British Communist Party and anyone suspected of acting in the interests of the Soviet Union. It was left to them to decide who that might be, and in the 1980s a whistle-blower named Cathy Massiter revealed that MI5 was spying on the miners' leader Arthur Scargill, on the leaders of CND, and on the future Cabinet ministers Harriet Harman and Patricia Hewitt, who ran what was then the National Council for Civil Liberties. In 1998, the Home Secretary, Jack Straw revealed that MI5 held files on half a million people, but only 20,000 files were "active" and of those, 13,000 were on British citizens.

Who are they watching now?

After the collapse of communism, MI5 needed a new role and concentrated on Irish terrorism. They had an agent named David Rupert high up in the Real IRA, who warned them in April 1998 that a bomb was likely to be placed in Omagh. Four months later, 29 died in the Omagh bombing. Eliza Manningham-Buller declined to meet relatives of the dead to explain why it was not prevented. Since 2001, MI5's first priority has been to keep watch on potential Islamist terrorists. In 2006, Manningham-Buller said they had 1,600 names on their list. A year later, Evans said it was 2,000. That figure does not appear to have changed.

Didn't MI5 let Mohammed Siddique Khan slip through their fingers?

The man who led the July 7 bombers was known to MI5, but they did not realise how dangerous he was. They claim that with more staff and more resources, they could keep a closer watch on people like Mohammed Siddique Khan. And, of course, MI5's success is measured by the number of suicide bombings they have prevented, which is never quantifiable. Mr Evans claimed that 86 convictions in two years have had a "chilling effect" on would-be terrorists, with the result that "they are keeping their heads down".

Should the Secret Service go back to being Secret?


* Agents who infiltrate terrorist groups are putting their lives on the line, and need secrecy

* Sooner or later, publicity will turn MI5 or its director into a terrorist target

* Events like Mr Evans's press briefing do not tell us anything useful anyway


* Secrecy has led to past abuses like the surveillance of Labour MP Harriet Harman

* Our taxes pay these people. We should know what they are doing and be able to hold them to account

* Publicity might improve the quality of people the secret services recruit