MEMPHIS — A federal prosecutor told a judge Friday that he will pursue the death penalty against a former prison guard charged with killing two Tennessee postal workers.

The defendant, 48-year-old Chastain Montgomery, lowered his head but showed no other reaction when U.S. Attorney Edward Stanton announced the decision during a hearing before U.S. District Judge Jon McCalla.

Executions of federal inmates are rare. Only three men have been executed under the federal death penalty since 1963 — Juan Raul Garza and Timothy McVeigh in June 2001 and Louis Jones Jr. in March 2003.

Federal prosecutors in West Tennessee last sought the death penalty in a 2009 carjacking-murder trial, the U.S. attorney's office said. Shannon Shields was convicted and ended up receiving life in prison that case.

Montgomery has pleaded not guilty to fatally shooting Paula Robinson and Judy Spray while robbing the Henning post office with his 18-year-old son in October 2010.

Prosecutors say he and Chastain Montgomery Jr., stole only $63, then shot the women in anger over the small amount.

The pair got away from the post office located about 50 miles northeast of Memphis and, according to authorities, robbed at least two Tennessee banks before the younger Montgomery stole a truck and was killed in a shootout with police in Mason in February 2011.

The elder Montgomery was arrested when he showed up at his son's shooting scene in the getaway car used in the post office killings, investigators said.

Defense attorneys have told the judge that they believe the elder Montgomery is mentally retarded, with an IQ of about 65. They are challenging a confession he gave authorities after his arrest at the scene of his son's death, arguing that he was not provided a lawyer during questioning and his statement was coerced by investigators.

Prosecutors met with Justice Department officials in Washington in January to discuss the case. The legal reasoning behind the decision is expected to be filed next week.

One justification for the death penalty will likely be that the two women were federal workers.

Defense attorney Michael Scholl said Montgomery's competency now becomes an even larger issue, because the U.S. Supreme Court has barred executing people with severe mental disability.

"It's going to amount to protracted litigation," Scholl said outside the courtroom. "It does make it a very expensive task for the taxpayers."

No trial date has been set. A hearing in September will address Montgomery's mental competency.