The bomb did not do the damage it was meant to

Six men are serving life imprisonment as a result.

But in light of the events of 11 September, a number of analysts are now questioning whether the attack was a warning that was never heeded.

"In many ways it was the opening salvo of al-Qaeda's campaign against the West," terrorism writer Simon Reeve told the BBC World Service's Analysis programme.

Next time I'll have more money, I'll come back and I'll bring those towers down 1993 bomb mastermind Ramzi Yousef

"Many of the individuals involved in the 1993 WTC bombing were connected in some way to Osama Bin Laden and al-Qaeda.

"This was their first attack."

He added that had things gone according to plan, the 1993 bombing would have caused greater devastation.

"The conspirators had actually planned to topple one of the Twin Towers into the other, while simultaneously releasing a cloud of cyanide gas," said Mr Reeve.

"What they were planning was really quite an apocalyptic terrorist attack."

Only financial restrictions had prevented the perpetrators from achieving their aim.

"They ran out of money. They didn't have enough gas canisters for the bomb, and some of the low-level members of the conspiracy - the foot soldiers - placed the bomb alongside the wrong support structure."

Lessons unlearned

After the attack, much of the US was stunned.

New York State Governor Mario Cuomo summed up the mood, saying: "Until now we were invulnerable."

But there was a limit to what could be learned directly - ironically because the group that carried out the attack had been so easy to track down.

"We encountered probably the dumbest terrorists we've ever encountered," recalls Neil Livingstone, one of the top terrorism experts in the US.

Former New York police chief Raymond Kelly says lessons should have been learned

Mohammed Salameh appeared in court only a week later, charged with the bombing after he tried to collect the deposit on the rental van used in the attack.

Three other men were arrested soon after.

But Mr Livingstone added that terrorists had learned a wider lesson - that the US was not invincible.

"They really believed our movies and popular culture that portrayed the CIA and the FBI as 10-feet tall," he said.

"What the attack in '93 demonstrated was that a relatively small group of relatively unsophisticated people could carry out a massive attack with very little money and nearly topple the World Trade Center."

Terror mastermind

Later in 1993, the FBI did uncover - and foil - several other planned attacks, including one on the UN building in New York.

But the man responsible for planning the WTC bomb, Ramzi Yousef, proved much harder to track down - and it was in dealing with him that experts contend the US should have first realised the strength of what they were now up against.

Yousef was only arrested in 1995, after a defector from his group in Pakistan informed on him.

Al-Qaeda finished what Yousef started

He was said to be planning further acts of terror, including assassinations on US President Bill Clinton and the Pope, as well as an attack on twelve airliners.

Elaine Shannon, who covered the investigation into the 1993 attack for Time magazine, told Analysis: "[Yousef] was very proud of what he did - he boasted that he was a terrorist."

"He was a Jihadist - he believes that the West is evil and must be brought down, and that this was a good place to start.

"When he got to Manhattan and the FBI circled over the World Trade Center, they said: 'See, it's still there.'"

"He said, 'Well next time I'll have more money, I'll come back and I'll bring those towers down'."

And it was Yousef's attack, Ms Shannon argues, that encouraged al-Qaeda to launch their own fully-fledged campaign of terror.

"The FBI's theory is that this was such an audacious act that al-Qaeda then sought out Ramzi Yousef and said: 'Do you want some more money, do you want to do some more imaginative bad things,' - and of course he did."