The terrorist attacks on New York and Washington marked a serious failure of intelligence, not least in the US. After the cold war, the intelligence agencies' main task was supposed to be combating terrorism and organised crime. Many are asking how the world's best-funded intelligence services could have failed to uncover a meticulously planned operation carried out on US soil.

One explanation is that the US has given up on old-fashioned spying. Although the annual budget for counter-terrorism has grown rapidly to some $12bn, the Central Intelligence Agency has run down its human intelligence capabilities. It has chosen instead to use technology: spy satellites, surveillance planes and electronic eavesdropping.

The enormous capacity of US intelligence agencies to gather raw information far outstrips their ability to analyse and interpret it. In any case, terrorists can easily cheat surveillance by using low-technology methods of communication. The perpetrators of last week's attack are thought to have used couriers rather than mobile phones or e-mail.

In order to combat small, loosely organised terrorist groups operating in America and abroad they must be infiltrated. The US needs to spend more of its estimated $30bn intelligence budget on recruiting and training field operatives and cultivating networks of agents and informers.

But before the US leads a general western rush back to cold war espionage, it is worth pausing to consider the implications.

First, one of the reasons the US has scaled back its human intelligence activities is public opposition to the human rights abuses sanctioned abroad by the CIA during the 1970s and 1980s. More human intelligence does not necessarily mean more of such transgressions but agents operating at arm's length are inevitably under weak control. As the Iran-Contra affair demonstrated, there is always a temptation to break the law in order to pursue national interests.

Second, there needs to be much more sharing of information between governments and closer judicial co-operation if the extra intelligence is to be of use. Terrorists should not be allowed to exploit national sovereignties to avoid capture. The US must do all it can to get access to Russian and Pakistani intelligence on Afghanistan, for example.

Third, the US will inevitably have to do more spying at home if it is to infiltrate terrorist groups operating there. Such a step requires appropriate legal safeguards, introduced after proper debate. Filling the gaps in anti-terrorist intelligence is essential; but it must be done with care and thought.