HAMPTON, N. H., Nov. 24-A mysterious anti-religious fanatic, who writes letters declaring churches are rackets for collecting money, is being sought in connection with the incendiary fire that shortly after vespers tonight turned the typical New England, white-frame Hampton Congregational Church here into a mass of billowing flames and smoke, and for the third time in a little more than two years, half destroyed the edifice.

The fire started 30 minutes after its pastor, the Rev. Floyd G. Kinsley, had locked the church to conduct the services at the Advent Church nearby as guest pastor.

From the character of the fire it was apparent that the flames were started in the organ of the church. The damage to this instrument was estimated at $5,000, while the damage to the pews, the ceiling and the semi-balcony in the rear of the auditorium is believed to involve another $10,000.

The church had just completed repairs costing $10,000 from the immediately previous fire in September, 1939. The fire before that, in November, 1937, is believed to furnish the clue to the arsonist in this case. For at that time the Rev. Herbert Walker, then pastor and now pastor-emeritus, received what were termed "poison pen" letters denouncing churches as a racket.

The investigation at that time yielded information leading to an alleged unbalanced suspect, but the Rev. Mr. Walker, authorities tonight said, was loath to prosecute. It was some time after the receipt of these letters that the church had its first fire. Officials are said to be on their way from police and fire authorities to track down the burner of churches. The fire in 1937 started in a closet, the one in 1939 began in some place in the church not determined,and today's fire was carefully started in the organ, although authorities did not say whether combustible materials were employed to hasten the flames.

The church authorities have had the building covered by insurance.