The murder trial of former Philadelphia Police Officer Ryan Pownall, which had been scheduled to begin Monday, has been delayed indefinitely as the District Attorney’s Office continues to challenge the state’s use-of-force law for police — a key issue in determining whether Pownall’s on-duty shooting in 2017 was a crime.

Common Pleas Court Judge Barbara A. McDermott on Monday agreed to postpone the trial until an appeals court weighs in on a request by the District Attorney’s Office to alter jury instructions about when officers are legally permitted to use deadly force on the job. McDermott did not set a new trial date, and prosecutors and Pownall’s defense attorneys did not say how long they believed the pretrial legal sparring might take.

At issue is the law governing when police are justified in using deadly force. The District Attorney’s Office said in a motion last month that it believes the current statute is unconstitutional because it allows officers to fire their guns in order to prevent the escape of a fleeing felon.

In her motion, Assistant District Attorney Tracy Tripp asked McDermott to alter the way she instructs jurors about the law. McDermott declined to do so, writing in an opinion that she had “no authority to summarily rewrite portions of a criminal statute.”

Pownall’s attorneys have argued that the current stance by prosecutors is an acknowledgment that their case is not supported by existing law.

One of Pownall’s lawyers, Fortunato Perri Jr., said after Monday’s hearing that the District Attorney’s Office had chosen to ignore the law when charging Pownall and is now seeking to change it to have a better chance to win at trial. The defense has maintained that Pownall’s actions were legally justified.

Pownall is accused of shooting David Jones during a traffic stop in North Philadelphia in June 2017. Police have said that he and Jones struggled over a gun in Jones’ waistband, and that Pownall fired after Jones dropped the gun and began running away. Pownall was fired over the shooting in September 2017.

District Attorney Larry Krasner charged Pownall a year later, the first time in two decades that a city cop had been prosecuted over an on-duty shooting. Krasner had been outspoken as a candidate about the lack of such cases brought by his predecessors.

Prosecutors have appealed McDermott’s ruling about modifying the jury instructions in the case. Perri and Tripp each said they were not certain how long it might take Superior Court to make a decision.

Tripp also declined to say Monday what the District Attorney’s Office would do if the appeals court declines her request.

McDermott allowed Pownall on Monday to be taken off house arrest while awaiting trial on third-degree murder and related counts.