Reporting from Beirut — The chief of Lebanon's domestic security forces had a warning for the Hezbollah commander: "You've been infiltrated."
With that, Achraf Rifi, head of the U.S.-backed Internal Security Forces, handed over evidence showing that two trusted, mid-ranking Hezbollah commanders were working as informants for Israeli military intelligence, said a high-ranking Lebanese security official with knowledge of the April 2009 meeting.
Wafiq Safa, the security chief for the powerful Shiite Muslim militia and political organization, was silent.
"They were shocked," said the security official, who spoke on the condition of anonymity because he wasn't authorized to speak on the subject.
Things moved quickly after that. The Hezbollah commander called Rifi the next day to assure him that the militant group would "take care of" the alleged infiltrators, who were never heard from again, the security official said.
A monthlong war between Hezbollah and Israel ended four years ago, and Israel's occupation of southern Lebanon ended a decade ago. But a clandestine intelligence war between the Jewish state and the Iranian-backed militant group continues unabated, officials and security experts say.
Now, a strengthening Lebanese government is helping Hezbollah bust alleged spy cells, sometimes using tools and tradecraft acquired from Western nations eager to build up Lebanon's security forces as a counterweight to the Shiite group, which since a 2008 power-sharing agreement has been a member of the governing coalition.
Although security officials here say they're using newfound tools to ferret out spies watching Hezbollah, just like they would against anyone attempting to infiltrate the country, Western observers express concern.
"There are deep Israeli worries that anything the West gives the Lebanese armed forces and the Internal Security Forces could be used against them," said Mara Karlin, a former Lebanon specialist at the U.S. Defense Department, now a researcher at the Johns Hopkins School of Advanced International Studies.
The United States and its Western allies play a delicate balancing game in Lebanon. Since 2006, Washington has given nearly $500 million in military aid to Lebanese security forces and has allocated $100 million for 2011, making Lebanon the second-largest recipient of American military aid per capita after Israel.
U.S. Assistant Secretary of Defense Alexander Vershbow met officials in Lebanon on Monday, emphasizing that continuing U.S. aid and training would allow the army to "prevent militias and other nongovernment organizations" from undermining the government.
The use of sophisticated equipment in the foiling of alleged Israeli spies may be the first concrete illustration of the U.S. dilemma. According to Lebanese officials, Israeli analysts and a Western diplomat who spoke on condition of anonymity, Lebanon has redirected for use against Israel signal-detection equipment donated by France and intended to fight Islamic militants.
"The technology used with Fatah Islam was used to detect Israeli spies and collaborators in Lebanon," said retired Col. Kamal Awar, a U.S.-trained former member of the Lebanese Special Forces who now publishes Defense 21, an Arabic-language military journal. "They discovered they were talking with the Israeli guy on the other side of the border."
The U.S. military has also contributed to the Lebanese security forces' communications abilities. Israeli analyst Ronen Bergman, author of "The Secret War with Iran," who is writing a book about the history of his country's intelligence efforts, said the U.S. gave Lebanon's army sophisticated electronic equipment that allowed it to identify and trace even encrypted communications.
But there is no evidence that the training and equipment have been used to foil the intelligence operations of Israel, a major American ally.
Israel and Lebanon have long claimed counterintelligence coups and thwarted alleged traitors.
In 2008, Israel charged Sgt. Maj. Lovai Balut of Military Intelligence Unit 504 of passing on information to Hezbollah, according to the Jerusalem Post. In June, the Israeli army arrested a soldier and several civilians accused of spying for Hezbollah and smuggling drugs into the Jewish state.
But over the last two years, Lebanon's security forces may have conducted one of the most extraordinary counterintelligence sweeps in the annals of espionage. Dozens of alleged spies have been arrested in Lebanon on suspicion of sending information to Israel on the whereabouts and movements of Hezbollah and other enemies of the Jewish state.
The broad range of suspects suggests a widespread effort by Israeli security forces to infiltrate Hezbollah, which Israel views as a severe threat to its national security.