Serbian investigators remain unconvinced, saying police found traces of suspected explosive materials on the cargo.
The Lebanese army said the missiles were being sent from Beirut to the American company that produced them.
It said the return was in accordance "with administrative and legal measures after the training ended."
But the Serbian public prosecutors' office said today in a statement emailed to The Associated Press it was investigating findings by border police that the packages "are suspected to contain explosive materials."
"We hope that our expertise will be finished this week," prosecution spokesman Ivan Markovic said, adding that the final report could take time "because some of the information is located in foreign countries."
Serbia's N1 television has said the packages, with two guided armour-piercing missiles, were discovered Saturday by a sniffer dog after an Air Serbia flight landed at Belgrade airport from the Lebanese capital, Beirut.
Serbian media said documents listed the final destination for the AGM-114 Hellfire missiles as Portland, Oregon, with a stopover in London.
They are also launched from drones in the US fight against terrorism.
The practice version of Hellfire is fitted with an inert warhead without explosives.
A US official familiar with the situation, speaking on condition of anonymity because he wasn't authorized to discuss the case, said the missiles are "inert training dummies."
"There was no explosive residue, no warhead and no engine," he said.
These are formally called "captive air training missiles" and they had been sent to Lebanon by their manufacturer, Lockheed Martin, for the Lebanese armed forces to practice mounting them on their Cessna planes, according to the official.
