A plainclothes D.C. police officer who was wounded Thursday night by two other officers who failed to recognize him after he called for assistance in making a drug arrest remained in serious condition yesterday as an investigation continued.

Ricky Graham, 25, a member of the D.C. force for less than a year, was working for the first time with the police casual clothes unit. He was reported to be in serious but stable condition in the intensive care unit of the Washington Hospital Center.

One of two bullets that struck Graham pierced his colon, liver and large intestine, according to a police source, and the other struck him in the left thigh. A police source said Graham was not wearing a bulletproof vest, which are available to all officers but are not required garb.

The shooting occurred about 9 p.m. outside a small grocery store near 25th Street and Minnesota Avenue SE in an area where police say illegal drugs such as PCP are openly bought and sold and crowds often gather.

Police officials have declined to comment on the incident until the investigation is completed.

Graham, who joined the force last April and was assigned to the 7th Police District, had been working as a uniformed officer for several months, but offered to work a plainclothes detail Thursday night when other officers called in sick, according to police sources who asked not to be identified.

"He had some old clothes in his locker, and he volunteered," according to one source.

According to police arrest reports filed in D.C. Superior Court, Graham and his partner, Carlos Hilliard, 23, were approached by a man who offered to sell them a substance wrapped in a foil package. When Hilliard identified himself as an officer, a struggle began and Graham radioed for help.

One police source gave this account:

When Sgt. David P. Davis and Officer Larry Cauley from the 7th District arrived, they saw Graham with his gun drawn. One of the officers fired two shots, and Graham fell with his revolver still gripped in his hand. He was ordered to drop the weapon, and when he failed to do so, one of the officers fired another shot.

During a televised interview after the shooting, Police Chief Maurice T. Turner Jr. said Cauley and Davis heard shots before they fired their weapons. Three witnesses interviewed by a reporter Thursday night said they heard a total of three shots.

After an incident 15 years ago in which a plainclothes officer was mistakenly shot and killed by a police sergeant in Stanton Park in Northeast, plainclothes D.C. police officers adopted certain visual cues to make them more recognizable to fellow officers. These include distinguishing types of clothing and hand signals.

It remained unclear yesterday whether Graham had identified himself properly when the other officers responded to his call for help.

A man who was arrested after the shooting, Derrick A. Jones, 20, of 4810 Alabama Ave. SE, was charged with possession of PCP with intent to distribute. He was arraigned in D.C. Superior Court and ordered held without bond for five days. Court records show that Jones was on probation on an earlier PCP possession conviction.

Police officials said Davis and Cauley have not been placed on administrative leave, and it may take investigators a week to complete interviews with witnesses. Staff writer Elsa Walsh contributed to this report.