Baltimore Officer William Porter, right, and his attorneys, Joseph Murtha, left, and Gary Proctor. An appeals court is considering whether Porter can be forced to testify against officers charged in Freddie Gray’s death as he awaits retrial in the case. (Bryan Woolston/Reuters)

Prosecutors who are forcing William G. Porter to testify against co-defendants charged in the death of Freddie Gray are using the officer as a “designated whipping boy” in their “thirst to convict others,” Porter’s attorneys argued.

The accusations were part of the latest round of legal briefings to a Maryland appeals court as Porter’s attorneys seek to prevent him from taking the witness stand against Officer Caesar Goodson Jr. and Sgt. Alicia D. White as he awaits retrial.

Porter’s first trial in December ended in a hung jury. A Baltimore City Circuit Court judge this month ordered Porter to testify against White and van driver Goodson, but the officer has appealed.

In papers filed Tuesday with the Maryland Court of Special Appeals, Porter attorneys Gary Proctor and Joseph Murtha underscored their concerns with making the officer testify. Despite prosecutors’ promises that they will not use Porter’s statements against him at a retrial, defense attorneys say the forced testimony would hurt Porter’s chances for a fair trial, taint future jury pools and put him at risk if federal charges are filed in Gray’s death.

Immunity is typically granted in plea deals and for grand jury testimony, but legal experts said this would be the first time in Maryland that it would be extended to someone facing trial.

“There are witnesses, and there are defendants with pending homicide trials,” Porter’s attorneys wrote. “It’s time to tell the state that never the twain shall meet.”

Prosecutors have said they cannot win convictions against Goodson (who faces the most serious charge, second-degree murder) and White (who faces manslaughter and other charges) without Porter’s testimony.

Porter, who faces manslaughter and other charges, was at five of six stops a police van made during the April arrest and transport of Gray, 25. Gray suffered a serious neck injury in the back of the van and died a week later, sparking protests and riots.

The state said officers were responsible for Gray’s death because they did not buckle him into his seat in the van and failed to call for medical assistance when he asked for help. But defense attorneys argued that Gray showed no immediate signs of distress and that arrestees were rarely strapped in while riding in the back of police wagons.

Porter’s attorneys said they are worried that prosecutors will attempt to use the officer’s forced testimony against him later, regardless of their promises of immunity.

Any statement the officer gives that may be inconsistent with previous testimony could be considered perjury, which is not protected under any sort of immunity and could prevent him from testifying at his own trial, Porter’s attorneys said.

The state “alleged that Officer Porter lied and attempted to cover up facts when giving a statement to police officers, and when taking the stand in his own defense,” Porter’s attorneys said. “Effectively, the State wishes to compel Porter, through the farce of a grant of immunity, to lay a foundation for evidence that the State has deemed as constituting an obstruction of justice and perjury.”

Prosecutors must file a legal response to Tuesday’s brief next month. The Court of Special Appeals is scheduled to hear the matter March 4.

Officer Edward Nero — who has been charged with second-degree assault, misconduct in office and reckless endangerment — is the next defendant set to go to trial, on Feb. 22.