The case against 12 Muslim men involved in what Gordon Brown described as a "major terrorist plot" amounted to one email and a handful of ambiguous telephone conversations, it emerged last night after all the men were released without charge.

Eleven Pakistani students and one British man were freed after extensive searches of 14 addresses in North-west England failed to locate evidence of terrorist activity, according to security sources. Police did not find any explosives, firearms, target lists, documents or any material which could have been used to carry out an attack.

Last night Lord Carlile, the reviewer of terror laws, said he would be heading an immediate "snapshot" investigation into the arrests.

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The Home Office said it would deport the 11 Pakistani men, who are aged 22 to 38 and were in Britain on student visas, because the Government believed they represented a threat to national security.

According to security sources, the operation was launched after the interception of telephone calls and emails which pointed towards a bombing campaign orchestrated by al-Qa'ida. But yesterday a senior Pakistani defence official said the British authorities had failed to consult them adequately before carrying out the arrests and greater co-operation would have avoided "embarrassing mistakes" for the British Government.

The official said: "They really started asking for our help properly after the arrests had been made over here. We have not arrested anyone in Pakistan, the families of these men or their friends, because we could not find anything against them, and we have a big file on terrorist suspects."

It has also emerged that there were disagreements among the various British security agencies about the way aspects of the operation were conducted.

Muslim groups and lawyers for the 12 men arrested two weeks ago in connection with the alleged terror attack plot on the North-west said the police operation had been a fiasco from start to finish.

The only British man questioned for 13 days in connection with the alleged plot may have been held on the strength of a single "cryptic" email, it emerged last night.

Lawyers for Hamzah Khan Shenwari, a 42-year-old security guard and delivery driver from Cheetham Hill, Manchester, disclosed that he had entered Britain nine years ago and was granted political asylum following his treatment at the hands of the Taliban. The Pashtu-speaker, who comes from the North West Province of Pakistan, where his family and children still live, was questioned over the course of 14 hours at a police station in Coventry although no substantial evidence was put before him, it was claimed. He claims he suffered bruises and a cracked rib during his arrest. Police believed that he was the "teacher" of the group because he was the oldest, his lawyers said.

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His solicitor, Sawar Khan, said: "He was elated this morning but his words to me were 'it is going to be difficult for me'. Even though he has been released without charge how will he be treated now? Most of the men come from north-west Pakistan but where they come from or what faith they are should not be a deciding factor in anything."

Lawyers for the men are furious that Gordon Brown and the Home Secretary made statements on the alleged plot during their detention period. Mr Khan said it was regrettable that, even though his client had been released, the Chief Constable of Greater Manchester Police had again talked up the threat of an unspecified terrorist attack.

He said there were question marks over the initial detention period of his client between 8 and 14 April. He was eventually released after Mr Justice Blake, who heard the application for a third extension of custody via a video link in the High Court, expressed "grave concern" over the progress of the investigation.

Mr Khan said Mr Shenwari was not seeking publicity or money. Police appeared to have few definite facts during the course of the questioning, it was claimed and no surveillance evidence was put to him. "A lot of the questions were repeated – he was asked whether he was involved in the preparation or instigation of terrorism."

Mr Shenwari, who does not own a computer, was asked by police about an email sent by one of the arrested men although the contents of the message were never made available to his defence team. It is understood that the email was cryptic and open to misinterpretation if taken out of context.

Mohammed Ayub said he would consider lodging an appeal for asylum on behalf of his clients, Sultan Sher, Mohammed Rizwan Sharif and Mohammed Umer Farooq, who were held at a police station in Bradford where they underwent up to six hours of questioning a day.

Mr Ayub said his clients, who all came from the North West Frontier territories of Pakistan, faced serious dangers should they be returned to their homeland.

"They face an uncertain future in Pakistan now they have been labelled as being engaged in potential terrorist activity," he said. "They cannot go back to living normal lives in Pakistan. The authorities may be obliged to put them into custody where they may be subjected to torture," he said.

"They were sustained by the belief that they were not involved in any wrongdoing and they maintained their innocence throughout. What put the police on to them in the first place? That is a very difficult question," he said.

During the interviews, Mr Ayub said no specific evidence had been put to his clients – no video footage or surveillance photographs were shown to them, though they were asked directly whether they were members of al-Qa'ida. Last night, 10 of the men were being held at detention centres. Their lawyers have seven days to present their cases to an appeals panel.

The North-west arrests: Key questions

When were the men arrested?

Manchester police officers were rushed into making the arrests after Bob Quick, then Britain's senior counter-terrorism officer, accidentally displayed information prejudicing the operation. Officers raided homes and universities across the North-west of England on 8 April. Twelve men were arrested under the Terrorism Act 2000. Mr Quick later resigned.

How long were they held?

One was released to the UK Borders Agency while the remaining 11 were questioned by police. Under terror laws suspects can be held without charge for up to 28 days. By yesterday all 12 had been released, two weeks before the maximum time limit expired.

Why didn't the police keep the men in custody longer?

The Terrorism Act includes judicial safeguards to ensure that the police are not simply detaining people while conducting "fishing expeditions" for evidence. The first time the police sought judicial permission was after 48 hours of detention. But they had to go back to court to authorise detention every week. After 14 days' detention the Crown Prosecution Service asked further permission from a High Court judge to keep the men in custody. It is understood that at this point the judge expressed concerns about the quality of the evidence being used to justify their detention. The investigation had clearly made little progress and the men were released.

What did the judge have to decide?

The court must be satisfied that there are reasonable grounds for believing that further detention will help obtain or preserve relevant evidence. But the judge must also be happy that the investigation is being conducted "diligently and expediently".

What happens next?

The Home Office says it will deport the 11 Pakistanis on grounds of national security. It is understood that the men want to carry on their studies in the UK and their lawyers are expected to go to court to contest this action.

Who will win?

It's hard to say. The Home Office must show that the men's presence in the UK is not conducive to the public good or that they represent a threat to national security. Their lawyers will argue that no evidence has been provided to support such a claim. The case is expected to end up before the Special Immigration Appeals Commission (Siac) which can hear evidence in secret.

What are they likely to say?

In the past Siac has blocked deportations to countries where the suspects face possible torture or execution. Now that the 11 students have been arrested on suspicion of what Gordon Brown described as a "major terrorism plot" they will almost certainly be of interest to the Pakistan security forces.

So what threat do they face upon their return?

The Pakistan police and security forces are known to have tortured terror suspects, some of whom are British citizens. Pakistan also retains the death penalty. Now that the names of the students are known to the Islamabad authorities the men may wish to pursue claims for asylum in the UK on humanitarian grounds.

How long will this case take?

It could go on for years. Several Algerians are still being held in the UK five years after they were accused of links to terrorism. It has taken that long for the House of Lords to rule that they can be safely deported.

Robert Verkaik