The evidence presented against Mr. Samaha has been considerable. Security officials told reporters that he had been caught with explosives that he had driven in from Syria after making plans to target “big crowds” and Sunni leaders who support the Free Syrian Army. (Mr. Samaha initially confessed, then recanted.)

A senior security official said in an interview that the evidence included about 90 minutes of video footage showing Mr. Samaha meeting with an informer whom he had hired to carry out the attacks. In two videos, Mr. Samaha described his plan in general terms, the official said, and in a third he can be seen in a Beirut parking garage, transferring more than 200 pounds of explosives from his car to the car of the informer.

The Lebanese official, who asked not to be identified because the courts have not made the evidence public yet, added that the final recording shows Mr. Samaha paying the informer $170,000. That was how much the Syrians had promised, he said.

While the Shiite response has been relatively muted, Hezbollah defended Mr. Samaha until investigators described the evidence. Since then, the organization has said virtually nothing, indicating that at least in this case Hezbollah’s agenda diverges from that of its longtime patron in Damascus.

Because Hezbollah’s legitimacy stems in part from its role as a leader in the resistance against Israel and its allies, “they are aware that if an internal Sunni-Shia conflict happens, they will lose their role as the resistance,” said Talal Atrissi, a political analyst and Hezbollah expert in Beirut. “They would be dragged into an internal war and lose that role.”

The abduction of dozens of Syrians, however, suggests that Hezbollah is not willing to stand idly by. Analysts and security officials said the kidnappings would probably not have occurred without a green light from the organization.