The world has become a smaller place following the terrorist attacks of 11 September.

But the serious task of investigating the perpetrators of the recent attacks requires painstaking cross-border scrutiny of bank accounts and evidence.Never has the issue of how easy it is to acquire information from other states been so important.In the UK, mutual assistance began formally in 1990 with the Criminal Justice (International Co-operation) Act.

Before that legislation , there had always been international informal co-operation between police forces, but the Act specified how the UK would support other countries' requests for assistance.There are five main types of assistance available: provision of evidence and witnesses for proceedings abroad; voluntary or compulsory seizure and search assistance; confiscation and restraint; service of process; and transfer of prisoners.Christopher Murray, head of crime at City firm Kingsley Napley -- who has co-authored the only law book on the subject and is a member of the Law Society's criminal law committee -- says: 'Half a century ago crimes were committed and investigated in one country.

Things have changed.

At the press of a button, funds can be transferred across the world from as many countries as you want.'Mutual assistance, he explains, is one way of trying to ensure that the investigatory powers keep on top of criminals.Mutual assistance ranges from the mundane -- French police investigating a traffic offence committed by a UK driver who has returned home -- to massive trans-national investigations.Mr Murray says the attacks of 11 September have changed the equation: 'We don't know, but we can assume that the US is making masses of requests to foreign inve stigators.'They will be helped in the UK by the progress that has been made with mutual assistance since the introduction of the enabling legislation in 1990.

Mr Murray says the UK is now recognised as being 'in the front rank internationally in its mutual assistance abilities'.Procedurally, a mutual assistance request starts life as a formal diplomatic request from a foreign state or central authority to the British Home Office.Within the Home Office, the mutual assistance unit -- headed up by former barrister Lorna Harris with a deputy and 16 case workers under her -- decides whether assistance should be granted, and which UK body should deal with the request.This may well be a local police force.

But in more serious cases, such as the Finninvest requests made by the Italian government in the 1990s in respect of an alleged fraud involving current prime minister Silvio Berlusconi, it goes to the Serious Fraud Office (SFO).Helen Garlick, the former barrister who heads the SFO's mutual assistance unit, says the unit is the victim of its own success.

Once section 2 of the Serious Fraud Act was amended in the mid 1990s to allow the SFO to use its powers in response to requests from foreign countries, the stream of requests has steadily increased.

Although the requests all come from the sifting process of the Home Office unit, this gives the SFO less time-wasting sorting out unmeritorious claims, Ms Garlick says.But direct requests to the SFO have been posited as a way forward, and Ms Garlick says: 'Since 11 September, a number of issues which were on the backburner have been given added influence.'The Finninvest case clarified the 1990 Act in determining the level of co-operation permitted.The smoothness of the process often depends on the state requesting it, and the system of law upon which it is based, Mr Murray explains.

For example, common law jurisdictions -- because their tests and procedures tend to be similar -- have a good record of assisting each other.

The same is true of continental civil law countries when they assist one another.

However, when the request crosses the legal cultural boundaries, problems can appear.For example, the French have made requests to the British in recent years to interview coach drivers in cases where English coaches have crashed in France.

But in the UK there is no obligation on anyone to answer questions under domestic law, and there are no powers of arrest under mutual assistance law, so French attempts have been stymied.To add to the problem, foreign countries frequently spend time and money before finding domestic UK law will not help.Toby Graham, a fraud partner at City firm Taylor Joynson Garrett, who has acted on mutual assistance cases for Transparency International and made submissions on the issue to the government's inquiry on corruption earlier this year, explains that cases can also be bogged down by judicial review.He says that three years ago, when the former government of Pakistan made a request for UK evidence for use in its trial of Asif Zardari -- former Pakistan president Benazir Bhutto's husband -- there were delays caused by judicial reviews of the then home secretary's decision.This has been reflected again recently in the current mutual assistance case being brought by the government of Nigeria, in its attempt to get documentation relating to the alleged theft of billions of dollars by former Nigerian dictator Sani Abacha.One judicial review case has now been rejected in that action, but the Home Office is believed to fear that another attempt t o challenge the home secretary's decision will be made further down the line.Moves are now afoot in the Home Office to make such judicial reviews more difficult for parties to bring.

Mr Graham explains that one way this could be done is to lower the requirements of reasonableness in the Home Secretary's decision-making, so that the threshold for showing he is ultra vires is higher.Other aspects of mutual assistance are also improving.

Organisations such as the European judicial network enable authorities to take efficient soundings on the likelihood of success of requests, and -- more importantly -- there are now liaison magistrates working within each of the European states.For example, in the Home Office mutual assistance unit, there are French, Italian and German liaison magistrates who can advise their governments on the best means of satisfying requests, and block requests which would be futile.However, amid the optimism over the future for mutual assistance, there remain some danger areas, Mr Murray says.One paramount principle of mutual assistance is that in complying with requests, no powers greater than those available in domestic law should be offered.But this has already been exposed to ambiguities, Mr Murray says: 'In the UK there are two levels of permission required for search warrants.

One -- which does not include privileged information -- is made through magistrates.

But permission for search warrants for documentation which is, for example, client privileged, must be obtained from a Crown Court.Mr Murray continues: 'However, if for example the FBI makes a request for documentation which would ordinarily require a Crown Court warrant in the UK, they may have the warrant granted by a magistrates' court convened under the 1990 Act and given special evidence-gathering powers.'This leads to a second danger with mutual assistance.

This is the interrelation of different tests required to obtain evidence.

Say that the test for obtaining a search warrant is slightly easier in Poland or Russia than in the UK.

If the UK authorities allow the easier test to apply to a mutual assistance request from that country, Mr Murray says this could in time lead to police forces forum shopping so that investigations can be launched from the states where the tests are easiest, rendering ineffective certain protections afforded to UK citizens.Both Mr Murray and Mr Graham agree that the mutual assistance unit needs more resources.

Mr Murray says: 'It does a fantastic job, Lorna Harris heads up a department of extremely hard-working lawyers.

But there is a feeling that more funding should be given to it.

It's one thing for the government to say that international co-operation is important, but without tools and budget, this can seem hollow.'Whatever the future of the unit, Mr Murray is adamant that mutual assistance will become more important for all lawyers, as they increasingly have to consider that challenges to their clients could come increasingly from abroad.Meanwhile, as the US tries to track down the money and perpetrators of the September attack, the ability of all states to assist is under the spotlight, and lawyers in the Home Office unit, the SFO and specialist firms are playing a crucial role.