Pakistani police on Wednesday questioned an American who allegedly attempted to board a plane with ammunition and knives, after US authorities said an FBI agent on an anti-corruption taskforce had been arrested there.
Pakistani police officer Rao Anwaar declined to say whether the detained American was the FBI agent.
He said investigators had not received any written evidence proving the man worked for the US Embassy in Pakistan.
The American arrived in Karachi on May 1 and was detained on Monday when he was about to board a flight for Islamabad, Anwaar said.
He said officials found the man carrying ammunition and three knives, as well as electronic devices that were being examined.
"We have sent his laptop and other gadgets for forensic examination," Anwaar said. He declined to comment further.
A law enforcement official in the US identified the man as an FBI agent and said he was in Pakistan as part of a multi-agency, anti-corruption programme.
The official said the agent appeared to have made a mistake and that there was no indication he was trying to carry bullets aboard the plane.
The official said authorities were hopeful the situation would be resolved soon with the agent's release, Associated Press news agency reported.
The official, who did not provide the agent's name or age, spoke on condition of anonymity because of the diplomatic sensitivities of the case.
Pakistani television channels aired footage on Wednesday of police presenting the American in court on Tuesday, which allowed the investigators to question him for four days. The footage showed the man in handcuffs, wearing a blue, striped shirt while standing in the dock.
Washington needs Pakistan's help fighting al-Qaeda and stabilising neighbouring Afghanistan, as NATO uses Pakistani roads to supply its troops.
However, relations have strained over a series of incidents. CIA contractor Raymond Davis shot and killed two Pakistani men in Lahore in January 2011.
The US killed Osama bin Laden in the Pakistani city of Abbottabad in May 2011 and American forces accidentally killed 24 Pakistani troops along the Afghan border the same year.
US drones strikes in the country have also angered Pakistanis.