Auto body maker Pabco Co. said Friday it illegally modified at least 10,351 trucks -- four times the number it announced in December -- by adding equipment that made them too heavy.

Pabco, a unit of Mitsubishi Fuso Truck & Bus Corp., said the illegal modifications date back to January 1986 and were made after the trucks had already passed vehicle inspections.

Of the trucks, 59 were over 1 ton heavier than when they were inspected because the fuel tanks had been replaced by heavier ones and plywood or stainless steel had been attached to the floor upon customer request.

If the trucks carry excessively heavy loads, it could place undue strain on the brakes.

Nearly 40 percent of the 10,351 vehicles were built for Mitsubishi Fuso, 23.3 percent for Isuzu Motor Ltd., 22.2 percent for Hino Motor Ltd. and 14.5 percent for Nissan Diesel Motor Co.

In addition, the Kanagawa Prefecture-based maker is suspected of illicitly obtaining inspection certificates for another 9,891 trucks.

When the inspection scandal broke in December, the company claimed only 2,353 trucks had been affected over three years through last March. But that figure rose after the firm conducted an in-house investigation.

The Land, Infrastructure and Transport Ministry filed a criminal accusation against Pabco in December with Kanagawa Prefectural Police on suspicion of violating the Road Vehicle Trucking Law.

Pabco President Shoichi Shimoda told reporters the firm will contact the owners of the vehicles in question -- mostly freight companies -- through truck dealerships and other truck makers by letter and urge them to bring the vehicles in for retrofitting.

Shimoda said making additional modifications after inspection was a long-held business practice when trucks are in high demand.

Police are trying to find out who is responsible for the fraud.