There is no point rushing in and destroying evidence which may be there

Det Chief Sup David Tomlinson

Mr Tomlinson said about 35 detectives were working on the criminal investigation.

He said: "This is going to be a long and protracted inquiry.

"Evidence gathering from witnesses and the scene is going to be a painstaking process.

"There is no point rushing in and destroying evidence which may be there."

Speaking at a news conference on Monday, Mr Tomlinson said he had visited the site with recovery and disaster specialists.

Firefighters damp down at the �100m centre

He added: "It is clear at present that it is too dangerous to enter the site.

"At the moment an initial logistical plan is being drawn up as to how we can go about recovering any evidence safely."

The police were unable to give any further information about the 25 detainees believed to be missing.

Mr Tomlinson said: "I am also aware there is a lot of concern about people unaccounted for.

"We have said that until the site is thoroughly searched we cannot be sure that everyone escaped unhurt.

"But it is important to emphasise at present we have no evidence to suggest anyone has died in the fire."

Three inquiries

Separate inquiries by the Immigration Service, police and the centre's security company Group 4 are under way into the incident, which caused damage valued at �38m.

Officers were attacked when an argument erupted in the visitors' centre on Thursday night.

Six people, including two police officers, were injured in the riot.

Built last year to house 900 people, Yarl's Wood is Europe's largest immigration centre.

Group 4 said 123 detainees have been transferred to other sites, with 222 remaining in the undamaged area.