In a case that has inflamed relations with Washington, an American, Raymond Davis, confessed to shooting dead two Pakistani men in self-defence in broad daylight on the streets of Lahore.
An eight-day police remand expires on Friday, when the judge is expected to decide whether Davis should be transferred from police custody to judicial remand in prison, a precursor to a trial.
Sanaullah said the prosecution would seek murder charges against Davis.
"Investigators will complete their job tonight and tomorrow we will frame murder charges against him," the law minister told AFP.
"Another case of carrying unlicensed weapons would also be taken up against him."
The case has become a political time-bomb, with the government under enormous pressure to see Davis go on trial.
Washington has demanded Davis' immediate release, saying he acted in self-defence and has diplomatic immunity while local lawyers argue that diplomatic immunity can be waived for grave crimes.
US lawmakers on Tuesday also threatened to cut aid to Pakistan and Washington has warned that high-level dialogue is at risk unless Davis is freed.
The deaths have also sparked protests in Pakistan, where anti-American sentiment runs high and few are convinced that Davis was a normal diplomat
The United States has yet to confirm his name and precise job.
Police told AFP they recovered a Glock pistol, four loaded magazines, a GPS navigation system and small telescope from his car.
A spokeswoman for the US embassy said the detained American would have legal counsel at the court appearance on Friday.
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