A 23-year-old Southwest Washington man was arrested yesterday afternoon on a warrant charging him with felony murder in the fatal shooting of a District taxi driver last month, D.C. police reported.

Investigators identified the suspect as Robert Todd Shannon but declined to give further details of the investigation that led to the arrest. Shannon, of 1012 Third St. SW, was being held without bond last night pending arraignment today.

He was charged in the Aug. 21 killing of William A. Keefer, 64, of Bladensburg, who had worked as a cabdriver in the District for more than 40 years. Keefer was found slumped in his Dial cab at Sixth and M streets SW shortly after midnight after an area resident heard gunshots and flagged down a passing police car.

Keefer, who was shot in the back of the head, died 30 minutes later at George Washington University Hospital.

Police said at the time that they believed Keefer was killed during a robbery, apparently by a gunman who ran from the scene immediately after the shooting.

Investigators declined to say last night if the weapon used in the slaying -- believed to be a large-caliber handgun -- had been recovered or if additional suspects were being sought.

Keefer was the second D.C. cabdriver slain during an apparent robbery this year. In January, Douglas R. Thornton, 42, who also drove a Dial cab, was struck on the head with a blunt instrument and left to die in an Arlington parking lot.

Keefer's death prompted a renewed outcry from other taxi drivers and some cab company officials over what they considered the lack of protection afforded cabbies by the District.

Dial Cab President Daniel Smith said the city was not concerned about the drivers because there are no regulations requiring protective shields between drivers and passengers or two-way radios that drivers could use to call for help.

Smith said it was too expensive for individual companies to put in these improvements in the absence of city regulations requiring them for all cabs.

Keefer's cab had no radio or protective glass, Smith said. For those drivers with radios, Smith said, Dial has a system of emergency signals that alerts dispatchers, other drivers and ultimately the police, if a driver is in trouble.