Egypt is not prepared to hand over to Britain a biochemist detained in connection with the London bombings, security officials said Saturday as British investigators attended interrogation sessions.

Magdy Mahmoud Mustafa el-Nashar (search), 33, was arrested on Thursday in Cairo after British officials supplied his name to Egyptian authorities. British investigators in the northern town of Leeds reportedly found traces of explosives in the bathtub in his apartment.

An Egyptian security official, speaking on condition of anonymity because a final decision has not yet been made, said interrogating el-Nashar was a matter of "sovereignty" and would be carried out on Egyptian soil.

The investigation was ongoing, the official said, adding that there was not enough evidence to prove el-Nashar was involved in the attacks.

Egypt and Britain do not have an extradition treaty, but Egyptian officials have said they are cooperating closely with British and American authorities on terrorism issues, and in el-Nashar's case in particular.

El-Nashar had been teaching at Leeds University and returned to Egypt a week before the bombings targeted London's transport system on July 7, killing 55 people, including four bombers.

Metropolitan Police Commissioner Sir Ian Blair (search) said Friday he wanted el-Nashar brought to Britain.

On Saturday, Egypt's Prosecutor General (search) said the nation's constitution bans the extradition of Egyptian citizens in connection to crimes committed abroad.

In the absence of an extradition treaty with Britain, Maher Abdel Wahed (search) said the rule would be "quid pro quo"— reciprocal treatment — or diplomatic courtesy. His remarks were carried by Egypt's Middle East News Agency.