US terror threat remains

The Pentagon was badly damaged in the attack

Security is being stepped up throughout the United States amid fears of a continuing terrorist presence following last Tuesday's attacks.

As the investigation continued, US Attorney General John Ashcroft warned that associates of the hijackers could still be at large.

Some suspects have been transferred to New York for questioning

In order to ensure passenger safety Mr Ashcroft said more armed federal agents will be placed aboard commercial airliners.

Federal Bureau of Investigation Director Robert Mueller said that 49 suspects had now been detained in connection with the attacks for immigration violations - nearly double the number at the beginning of the weekend.

More detainees

The detainees included two men seized at a train station in Fort Worth, Texas on Thursday.

Identified as Ayub Ali Khan and Mohammed Jaweed Azmath, they were reportedly carrying $5,000 in cash and knives similar to those used in the hijacking.

Police across Europe are following up leads

Mr Mueller said that warrants have been issued for two more potential material witnesses, although he gave no details on the warrants.

Two other individuals are already under arrest as material witnesses and are being held in New York.

Another four people have also been transferred to New York for questioning, authorities said.

The FBI are currently trying to track down 170 people for questioning.

Appeal for agents

As the investigation fans out across the globe the FBI have received more than 50,000 e-mails and phone calls offering tips.

Mr Mueller has directed the US Marshals Service to assign more than 300 deputies to assist about 4,000 FBI agents already working on the investigation.

Demonstrating the difficulties facing the investigators Mr Mueller publicly appealed for people with a "professional level in Arabic and Farsi" to come forward.

Police and anti-terrorist experts Belgium, France, Germany and The Netherlands met in Brussels to discuss progress so far in EU investigations into the attacks.

In Germany a special police unit probing connections to three suspects in the attacks on the United States has searched a further four apartments, two in the northern city of Hamburg and two in the western town of Bochum.

Global effort

Italian stock market authorities said on Monday that they were investigating abnormal movements in share prices on the Milan stock exchange prior to the terrorist attacks on the United States.

News of the investigation comes after Defence Minister Antonio Martino said in an interview that he suspected that terrorists had speculated on the markets.

The Russian internal security service says it has found a computer compact disc during a raid in Chechnya which contains instructions for piloting Boeing aircraft similar to those hijacked for the attacks in the United States last week.

The chief spokesman of the Federal Security Service, Alexander Zdanovich, said it was found during a raid on a group which had contacts with Osama bin Laden.