Lyndon Baines Johnson Baker, 16-year-old son of former Capitol Hill aide Bobby Baker, was killed and a second youth fatally injured Thursday night when young Baker's car jumped a curb at a traffic circle in Northwest Washington and careened into a tree.

Sean Keegan, also 16, who was riding in the front seat with young Baker, died at 11:30 a.m. yesterday at Suburban Hospital in Bethesda.

Baker and Keegan were students at St. John's College High School. Baker lived at 3143 Oliver St. NW, and Keegan at 5209 Upton Terrace, NW.

Three other Washington youths were injured in the accident. All three were in the back seat when Baker drove onto Pinehurst Circle off Utah Avenue NW, and crashed into a tree on the circle.

D.C. police traffic investigators said yesterday that Baker was driving "at an unreasonable speed" at the time.

The others in the car were Mark Evans, 17, who was in the intensive care unit at Sibley Hospital in critical condition last night, Dario Presentado, 17, listed in critical condition at Suburban Hospital, and James Gerrety, 17, released yesterday at Suburban.

Baker was the youngest son of Robert G. (Bobby) Baker, former secretary to the Senate and the self-professed "Capitol Hill operator" for President Lyndon Johnson.

The elder Baker was convicted in 1967 of larcen, fraud and tax evasion charges. He served 18 months in prison.