The probe by the Tel Aviv fraud squad began several months ago and found evidence that Trojan horses, viruses designed to spy on computer systems, had been planted in computers of some of the country's top companies, an Israeli police representative said Sunday.

The officials the police are investigating work for firms including Israel's top mobile phone operator, Cellcom, and two subsidiaries of dominant phone company Bezeq Israel Telecom-- mobile phone operator Pelephone and the satellite television provider Yes.

All three companies issued statements saying they are cooperating with the police and had done nothing illegal.

A total of 18 people were detained for questioning including the security managers at Pelephone and Cellcom, who have since been released, and several private detectives suspected of planting the viruses, the police representative said.

No charges have been filed.

The chief financial officer of Yes is still being held, the police representative added.

"In this manner of industrial espionage, the private detectives gathered a great deal of information for their clients, who according to suspicions are rival companies," the Tel Aviv police said in a statement.

The police representative said that two people suspected of creating the Trojan horse, an Israeli who works abroad and his girlfriend, were being held in London and that Israel plans to request their extradition.

The Israeli police said they are working with police forces abroad.

Among the companies infiltrated by the Trojan horse are the Hot cable television group that competes with Yes, as well as foodmaker Strauss-Elite and the Rani Rahav public relations agency, whose clients include Partner Communications , Israel's second biggest mobile phone operator, police said.