DALLAS (AP) _ An undercover drug deal ended in the slaying of a 17-year police veteran, the fifth officer to die in the bloodiest year in department history.

A suspected drug dealer also was killed and two other suspects were seriously wounded in the shootout Tuesday night, police said.

Officer Lawrence R. Cadena, 43, was shot with an assault rifle, possibly an automatic weapon, as he bought cocaine in a convenience store parking lot, police said.

Cadena, who joined the force in 1971 and had been a narcotics officer since 1986, died at Baylor University Medical Center from wounds to the chest, abdomen and arms, said Sgt. Jim Chandler.

Police said they believe the shooting erupted after the suspected dealers attempted to rob the plainclothes officer. Witnesses reported hearing up to 11 shots.

Cadena was the fifth Dallas officer shot to death this year. Three died in the line of duty, more than in any other year; two were killed while moonlighting on security jobs.

A drug suspect, Hector Jacomino, 34, died at Parkland Memorial Hospital, emergency room supervisor Randy Blanchard said.

Two other suspects, Javier Surez, 17, and Fernando Fernandez, 19, were in critical and serious condition respectively, Blanchard said.

Police arrested two other suspects at a nearby apartment early today, police spokesman Ed Spencer said. Their names were not released.

''It is my understanding that it was a buy-bust situation - a drug buy - and we had more than one officer at the scene,'' said Police Chief Mack Vines. ''Instead of the drug deal reaching fruition, they were hijacked ... shots were exchanged and our officer was fatally wounded.''

Vines said other officers were in the area as backup for the planned drug bust but did not participate in the shootings.

''I heard three gunshots and then I saw a guy leaning on a Camaro. Three people were lying on the ground,'' said David Zwaska, 32 of Dallas. ''I was just passing by here when I heard the shots.''

Ali Dahii, another witness, said he heard up to 11 shots fired while he used a pay telephone near the convenience store.

Wan Kwak, owner of a food store across the street, said he heard the shots and then ran to door of his store to see the gunfight.

''Right up until that time, they fight each other with big guns right out on the street,'' said Kwak, 57. ''When I looked over on the street, people were running into each other and they were shooting at each other.''