TERROR PROBE POLICE STUDY IMAGES.

Officers are carrying out intensive searches of all the properties raided

Police scouring homes for evidence of a planned terror attack in the Manchester area are studying photographs found at one location, the BBC has learned.

Senior officers described claims that shopping centres and a nightclub were among potential targets as speculation.

Twelve men - 11 of them Pakistani and most of them students - are still being questioned over the alleged plot.

Gordon Brown has said Pakistan must do more to stop terrorism. Pakistan says student visa checks are up to the UK.

Police arrested the 12 suspects during a series of raids in Manchester, Liverpool and Clitheroe in Lancashire.

The planned operation had to be brought forward to Wednesday following a security blunder by Britain's anti-terror chief.

Assistant Commissioner Bob Quick had been photographed carrying documents with details of the alleged plot clearly visible. He resigned on Thursday.

Wajid Hasan: Pakistan is not responsible for Islamists coming to Britain.

Since the raids, forensic officers have been gathering evidence at 14 properties in north-west England and taking away vehicles for examination.

Senior officers insisted on Thursday that there was no intelligence pointing to any specific targets.

However, the BBC's Nick Ravenscroft said sources had confirmed that photographs of four popular Manchester locations were recovered during searches.

These were the Arndale and Trafford Centre shopping complexes, Birdcage nightclub and St Ann's Square.

"These may not have been at risk of any terrorist attack, though sources close to the investigation say that a very big attack was going to happen very soon, possibly within days of weeks," our correspondent added.

The fact that many of those arrested had entered the country as students has led to Conservative calls for the government to "step up" checks on visa applications.

Almost 10,000 Pakistani nationals were granted student visas in the year to April 2008 and the Home Office says it has recently introduced measures such as fingerprinting to tighten the system.

Educational institutions also have to demonstrate they are keeping track of foreign students.

John Tincey, of the Immigration Service Union, said a big problem was assessing the colleges and schools in Pakistan, and that many of the institutions were fronts for illegal immigration.

"Very often [they are] a normal house in a normal road where a couple of rooms are designated as the school, and basically there's very little evidence of any education taking [place]," he said.

Qasim Rafiq, from the Federation of Student Islamic Societies, says he is concerned about the rigour of the visa process for students.

"There is definitely a need to look at the processes that are currently in place, to check students coming abroad - from any country," he said.

"But I don't think that we need to respond and have a knee-jerk like reaction. I think we need to look at this very carefully."

The director of the Royal United Services Institute, Professor Michael Clarke, said certain overseas students represented a relatively easy target for the security services.

"You can bet your bottom dollar that the security services take a great interest in people travelling on student visas," he said.

"The problem they have is, have they got the numbers and the manpower to do the checks?"

Prime Minister Gordon Brown said he would speak to Pakistan's president to raise concerns about what he termed "increasing" terror links between the country and the UK.

"One of the lessons we have learned from the past few years is that Pakistan has to do more to root out terrorist elements in its country as well," he said.

But Pakistan's High Commissioner to the UK, Wajid Shamsul Hasan, said his country already did enough and was arresting people in anti-terror raids "every day".

"You have to do something more," he told BBC's Newsnight.

"If [the UK government] allow us to make inquiries... ask us to scrutinise people who are seeking visas, we can help them."

Bob Milton, a former Metropolitan Police Commander with 25 years of counter-terrorism experience, says al-Qaeda may be adopting new tactics.

He said: "This seems quite a logical step. Why not place sleepers in the UK - in the universities - who've come from Pakistan, would have been trained, indoctrinated in Pakistan.

"Send them over here and then utilise them to carry out attacks against the UK."

Community cohesion minister Sadiq Khan accepted visas were "our issue" and said student applicants were now being dealt with through a "points-based system".

"We've changed the rules so you've got to be applied to come to a registered university or college before you can come and you have your fingerprints taken and you are biometrically tested."

The Home Office says applicants are checked against "immigration, terrorist and crime watch lists".

However, Mr Khan admitted he did not know how many student visa applications had been refused on security grounds.

BBC NEWS REPORT.