There is no evidence that any of the London [Images] suicide bombers visited religious schools in Pakistan, a top official has said amid an investigation into whether attackers received aid or inspiration from local militants.

Education Minister Javed Ashraf Qazi made the comment on Wednesday night as authorities tried to determine the activities of two bombers of Pakistani origin who visited Pakistan in 2004.

The men, Shahzad Tanweer and Mohammed Sidique Khan, and two accomplices set off bombs in the London transport system on July 7, killing 56 people, including themselves.

• The London Blasts

After the attacks, Pakistan faced renewed criticism from some Western leaders and experts who said the Pakistani government had not done enough to curb Islamic extremism in some religious schools, or madarssas. Pakistan recently launched a crackdown that led to the arrests of hundreds of suspected militants.

"None of the London bombing suspects visited any Pakistani madrassa, and we have no evidence about it," Qazi told reporters.

Qazi also claimed that no Pakistani madarssa was giving military training to its students.

Some reports have said Tanweer visited two madarssas after travelling to Pakistan late in 2004.

Separately, a government official in Islamabad said on condition of anonymity that Pakistani authorities were trying to determine whether a possible ringleader in the failed attacks in London on July 21 -- Ethiopian-born Muktar Said Ibrahim -- had visited Pakistan.