Investigations into who was behind Tuesday's terrorist attacks in the U.S. began just hours after the first plane struck the North Tower of New York City's World Trade Center.

Within 48 hours some 4,000 special agents and 3,000 support personnel were assigned to the case, with about 400 FBI laboratory specialists deployed to examine the forensic evidence.

Almost immediately the finger of suspicion was pointed at exiled Saudi dissident Osama bin Laden and his shadowy group of followers known as al Qaeda, or "The Base."

He has been implicated in a series of attacks on and plots against U.S. targets, and also has a well-known grudge against the United States and its people.

On top of that there is the compelling evidence that men connected with him have targeted the World Trade Center before -- in a 1993 truck bomb attack that left six dead and more than 1,000 injured.

Bin Laden has denied that he was himself involved in those attacks but nonetheless has said he supports the actions of those who carried them out.

He has been living as a "guest" of Afghanistan's Taliban authorities for several years and fears that the U.S. may target the country for retaliation has caused international aid workers to stream out of the capital, Kabul. But what evidence is there linking him to Tuesday's horrendous events?

High pressure

Whilst pressure is growing for the U.S. to hit back against him, there will also be pressure to ensure that they pick the right target.

Publicly the Bush administration has refrained from naming anyone as a suspect.

Getting it wrong or striking without having the evidence to back up their case could undermine international support and erode any of the moral high ground Washington could claim.

U.S. Attorney General John Ashcroft said Wednesday that authorities were reviewing "numerous credible leads" as the investigation powered up to full speed.

He said four separate groups of terrorists may have been involved in the hijackings, with one group believed to have close ties to bin Laden thought to have crossed from Canada.

Officials have been quoted in several off the record comments as saying that they had intercepted telephone and email messages between known bin Laden associates talking about the attack.

There are also reports of officials investigating the possibility that the attacks were carried out by some sort of consortium of terrorist groups, of which bin Laden and Al Queda were just one part.

Terrorist kingpins

"This could have been the result of several terrorist kingpins working together," one law enforcement official speaking on condition of anonymity told The Associated Press.

"We're investigating that possibility."

Authorities are also reported to have been gathering evidence that some of those involved in Tuesday's attacks may also have been behind, among others, the bombing of the USS Cole off Yemen and the Millennium bombing plot on U.S. soil.

Speaking to the AP Wednesday, Sen. Charles Grassley, the top Republican on the Senate Finance Committee, said a briefing from law enforcement officials had left him with the impression of a well coordinated and widespread network of plotters.

"Most of it today points to bin Laden but the speculation at the end of the road is that he and his network were very much involved with Hezbollah, Fatah and other" terrorist organizations, he said.

As investigations continue, FBI officials have asked for assistance from intelligence agencies around the world that may help firm up bin Laden's involvement.

Pakistan, which has close ties to the Taliban leadership, has been asked for cooperation -- a request which appears to have received a rapid and favorable response.

The Indian government has also been asked to open their files on bin Laden, the Times of India reported Thursday.

'Suicide notes'

In the U.S., government officials say they have identified some 50 people thought to be behind the attack, at least 10 of whom they said remained at large.

Quoting "a source familiar with the investigation" Thursday's Los Angles Times said the other 40 suspects had been accounted for, including the 12-24 hijackers who died as the planes they commandeered ploughed into their targets.

The LA Times said agents who searched cars and apartments found suicide notes in New York that some of the hijackers wrote for their parents.

Another apparently strong trail of evidence lies in Boston, from where the two flights that crashed into the World Trade Center originated.

On Wednesday police in the city confiscated a car believed to belong to the hijackers -- they have confirmed that among items found inside was an Arabic-language flight manual.

Boston FBI officers say bin Laden has strong family ties in the city and a close-knit group of supporters.

Robert Fitzpatrick, former second-in-command at the FBI's Boston office, told the Associated Press Wednesday he had seen reports detailing scholarship funds at Harvard University that had been set up by one of bin Laden's brothers.

He said another relative owns six condominiums in an expensive complex in Charlestown, just outside Boston.

According to Fitzpatrick investigators have also interviewed drivers from a Boston taxi firm where two known associates of bin Laden once worked.

There are suspicions they may have had ties to airport baggage handlers, who in turn may have supplied weapons to the hijackers, the former FBI boss said.