A senior Israeli official says Jerusalem has evidence of extensive phone traffic between Lebanon and Burgas, Bulgaria, in the months leading up to the July 18 suicide bombing at the Burgas airport which saw five Israeli tourists and their Bulgarian bus driver killed.

The senior government source, who spoke to The New York Times on condition of anonymity, said Israeli intelligence had picked up several telephone calls between Burgas and Lebanon in the two months before the bombing.

The official said the chatter intensified significantly in the three days leading up to the attack.

“We know the sources in Lebanon,” the official said, adding that the other end of the line in Bulgaria was still a mystery. “They shouldn’t know that we know the numbers in Lebanon.”

Prime Minister Benjamin Netanyahu quickly blamed Iran and proxy Hezbollah for the bombing, saying it was the latest in a series of attacks and attempted attacks on Israeli and Jewish interests abroad backed by the Islamic Republic.

His assessment, backed by defense officials, was later corroborated by senior figures in the American security community.

The phone traffic between Burgas and Lebanon would seem to imply a connection to Hezbollah, a terror group that operates out of southern Lebanon. Hezbollah was also linked to the recent arrest of a man in Cyprus believed to have been planning an attack on Israelis.

Officials have still not made any public statement about the identity of the bomber, and Sofia is still investigating the incident.

Last week, Bulgarian intelligence released a computer-generated sketch of the suspected bomber, based on remains found at the scene of the attack, and asked for the public’s help in identifying him.