The attacker tossed a small bomb or hand grenade directly into the restuarant courtyard, which is known for serving alcohol to Westerners.
Multiple sources said that the attack was under investigation to see whether it was based on the terrorists having learned in advance of the agents' presence.
In Washington, meanwhile, officials downplayed the possibility that the attack specifically targeted the agents based on advance intelligence.
They cited the relatively small size of the attack -- one in which injuries to the agent's were relatively minor -- as part of their rationale. Also the explosive does not appear to have been tossed directly at the agents' table.
The top agent in the country, U.S. Embassy FBI attache Ray Biteski, suffered serious ear damage, an assistant attache reportedly suffered a concussion and two agents are reported as suffering minor injuries.
The two who suffered minor injuries were treated and released. Biteski and a second agent were med-evaced to Europe.
This was the first bomb attack on U.S. citizens or government personnel in Pakistan since a church bombing in 2002.