The brother of a Pakistani killed by a US consular vehicle in Lahore after a shoot-out which killed two other men went to court on Friday to demand the driver be arrested.
The incident has already racked up tensions between Islamabad and Washington, with the US demanding the release of the gunman, Raymond Davis, saying he was acting in self-defence and has diplomatic immunity.
Aijazur Rehman, whose brother Abadur was run over by a car sent to rescue Davis, is seeking the arrest of its driver, who has not been identified, and the surrender of the vehicle.
His lawyer Nauman Atiq told AFP that Rehman was not satisfied with the police investigation.
"Police is telling us that they cannot arrest the driver or recover the vehicle because they cannot go inside the consulate," Atiq said.
The case threatens to open a second front in the diplomatic struggle over the deaths, which have left the unpopular government in Islamabad under huge domestic pressure not to be seen as kowtowing to US demands.
But the Lahore High Court did not specifically address the issue of arresting the driver, Atiq said.
"The Lahore High Court chief justice Ijaz Ahmad Chaudhry directed the Punjab government that it should investigate the case strictly in accordance with the law," he told AFP.
"The chief justice also ordered the investigators to record the statements of the witnesses including the brother of the deceased man."
A court official confirmed the judge's order, and provincial assistant advocate general Waqas Qadeer Dar, representing the provincial government said he told the judge that the court's directions would be "fully complied" with.
Punjab law minister Rana Sanaullah has said that the government has written five letters to the US consulate asking for the driver and vehicle to be handed over, but so far they had not responded.
Islamabad is a close ally of the US in the war against Taliban militants in Afghanistan, but Pakistan's population of 167 million is strongly anti-American, angered by US missile strikes in the country's northwest.
Pakistani police have accused Davis of double murder and the Lahore High Court Thursday put off ruling on whether he had diplomatic immunity, threatening to prolong the crisis with Washington for another month.
Another court on Thursday remanded Davis in custody until March 3 in a separate case of illegal arms possession, the American making an appearance by video link from jail, said Punjab government prosecutor Abdul Samad.