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Substance found in Rome raid is cyanide derivative

Security at the U.S. Embassy in Rome remains tight after Italian authorities found suspicious maps and a cyanide derivative in an apartment raid.

ROME, Italy (CNN) -- A white substance found in a police raid that sparked fear of a terrorist attack in Rome is a cyanide derivative far less toxic than the pure form of the poison, Italian authorities said Wednesday.

In the Tuesday raid on a Rome apartment, officials found a white powder substance, a map marked with the location of the U.S. Embassy and a detailed map of the Italian capital's underground water system.

Authorities said the substance is potassium ferrocyanide, a chemical compound used in winemaking and gardening. The compound can form toxic gases if heated to a high degree or mixed with hot, concentrated acids.

Cyanide is a water-soluble poison that can be used as a chemical weapon and is difficult to detect and smell.

MORE STORIES On the scene: Alessio Vinci on the suspected threat

Italian media reports said the white powder was found in a container inside the apartment.

The raid resulted in the arrest of four Moroccans in Rome. The men remain under arrest and are being questioned about the substance and the two maps.

Authorities later said the water system map is available to any client of the city waterworks, but they are questioning the four men about why they had a copy of it and why the U.S. Embassy was circled on the other map.

Media reports said the men had been under surveillance for several days before police conducted Tuesday's pre-dawn raid.

As a result of the raid, Rome police told U.S. officials about the possibility of a security threat against the U.S. Embassy.

Embassy officials convened an emergency meeting Wednesday morning but would not comment further because of the ongoing investigation.

Security remains tight, but no new measures have been enacted to safeguard the embassy.

In Washington, State Department spokesman Richard Boucher would not answer questions, saying he could not offer any details about the investigation.

"We have been working with Italian authorities, and we'll continue to work with them as they investigate those who have been arrested," Boucher said.

CNN Rome Bureau Chief Alessio Vinci contributed to this report.