If this substance had been put in the water network it appears that it would not have been capable of causing any damage whatsoever

Italy's government official

But tests later showed that the substance was a form of potassium ferrocyanide that did not have enough cyanide to cause major damage.

"If this substance had been put in the water network it appears that it would not have been capable of causing any damage whatsoever," said Emilio del Mese, a government official.

US State Department spokesman Richard Boucher praised the Italian authorities for "their ongoing commitment to countering the terrorist threat" but denied that embassy staff had been at risk.

The four men, aged between 30 and 40, were detained on Tuesday in a raid on a flat in the Tor Bella Monaca area in the south of the Italian capital.

Police also found maps marking the capital's water supply network, as well as a hoard of false documentation and Islamic extremist propaganda.

Intelligence operation

Magistrates are reported to be outraged that news of the arrests has been leaked to the press.

They say it will jeopardise efforts to reveal who the men were working with.

Italian intelligence services are reported to have mounted a massive intelligence operation to find out how the al-Qaeda network functions.

Since 11 September, 20 people have been arrested in Italy on suspicion of having links to extremist groups.

Surveillance of the four men arrested on Tuesday has allegedly revealed links to four Tunisians on trial in Milan over purported ties with the al-Qaeda network.

A verdict in that case is expected on Friday.