Prince George's County police said yesterday it will probably be several weeks before they decide whether to charge a 280-pound man who they say might have killed a robbery suspect by sitting on him during a citizen's arrest.

Cpl. Diane Richardson, a police spokeswoman, said investigators are waiting for the state medical examiner to determine how the robbery suspect died and whether he was under the influence of drugs or alcohol.

"Really, the course of the investigation will depend on what the medical examiner says the cause of death is," Richardson said.

James R. Thompson, 37, of the 3800 block of St. Barnabas Road in Temple Hills, was pronounced dead Sunday morning at a Camp Springs 7-Eleven store after police arrived to arrest him in connection with the robbery of the convenience store.

Richardson said Thompson walked into the store and began stealing cigarettes and implied he had a gun. A customer in the store, whom police will not identify because he is a witness, intervened and wrestled Thompson to the ground.

The 6-foot-2-inch, 280-pound customer sat on the 5--foot- 2-inch, 150-pound Thompson until officers from the Morningside police department arrived, the Washington Post reported yesterday. Police then discovered that Thompson, who was carrying a concealed knife, was not breathing, Richardson said.

A spokeswoman for the state medical examiner's office said yesterday it could be several weeks before Thompson's autopsy is completed.

Abraham A. Dash, a professor at the University of Maryland School of Law, said he doubts that the customer will be charged.

"It sounds like there was obviously no intent to inflict harm," Dash said.

Dash said it is legal in Maryland for citizens to detain a suspect until police arrive, but they cannot use more force than necessary.