WASHINGTON, Dec. 18 — Offering their explanation for one of the most puzzling auto safety mysteries in decades, Firestone engineers today blamed a unique combination of tire and vehicle design, tire manufacturing and customer abuse for tire failures linked to the deaths of more than 100 Americans, almost all in Ford Explorers.

A four-month internal investigation by Firestone found that the tire failures had their roots in decisions as simple as whether to use rubber pellets or rubber sheets in certain manufacturing processes, and whether the grooves in the side of a tire's tread were vertical or slanted, the engineers said in an interview today.

The inquiry also found fault with Ford, asserting that by letting the Explorer's weight swell during the mid-1990's, the automaker cut into an already slender safety margin in the tires' load-bearing capacity. The engineers also blamed drivers for not keeping their tires properly inflated and for fixing flats incorrectly.

While Firestone's chief executive told Congressional committees in September that the company had made "some bad tires," the engineers' comments were the first time that the company has publicly acknowledged making specific manufacturing mistakes. Under pressure from Ford, and with the government moving to investigate, Firestone recalled 6.5 million tires for sport utility vehicles in August. The tires figure in more than 100 lawsuits seeking compensation for deaths, injuries and property damage when their treads peeled off. Similar tires have been implicated in dozens of deaths overseas.

The findings, which Firestone reported to federal regulators last week, mean that the company and its Japanese parent, the Bridgestone Corporation, are lashing their fate in the courts to Ford, which has sought to direct blame at Firestone and away from the Explorer's design.

The conclusions also absolved workers in Firestone plants of culpability for the tire failures, pinning any flaws in the tires themselves on the company's design and manufacturing processes.

Told what the Firestone engineers had said, Ford and a leading personal injury lawyer were quick today to question whether Firestone's investigation really went to the heart of the problems with the tires.

Ken Zino, a Ford spokesman, said that Firestone was wrong in focusing on the narrow margin between the tires' maximum weight-carrying capacity and the weight of the fully- loaded Explorer. Explorers equipped with other tires — including a smaller Firestone tire used in the early 1990's that had a slightly lower weight-bearing capacity — have not had similar failures, he said.

Lawyers pursuing suits over the tire failures objected to Firestone's conclusion that its design and manufacturing problems were confined to the recalled tires. C. Tab Turner, a lawyer in Little Rock, Ark., who has led the legal assault against Ford and Firestone, contended that millions of 15-inch and 16-inch Wilderness tires not covered by the recall were virtually identical to those that have been recalled.

Ford has also been examining whether the recall — which covers 15-inch ATX and ATX II tires — which were made until 1996 — along with newer 15-inch Wilderness tires produced in Firestone's Decatur, Ill., factory — has been broad enough. Mr. Zino said that while Ford was satisfied "for now," it was still reviewing the design of those Wilderness tires that have not been recalled.

Details of Firestone's findings emerged from a three-and-a-half hour interview today with three engineers whom the company made available on the condition that they not be identified, because of the pending litigation. See What Went Wrong.

Besides the technical analysis, their investigation identified shortcomings in the way Firestone handled what turned out to be crucial safety information. While Congressional investigators and personal injury lawyers have accused the company of ignoring problems for years, the engineers insisted today that Firestone's safety staff had simply failed to look at damage claims received by its sales staff.

Firestone's database on damage claims was moved to Nashville, the headquarters of Bridgestone's American unit, after Bridgestone bought Firestone in 1988. But the database on warranty adjustments, which the safety staff regularly used, remained behind at Firestone's former headquarters in Akron, Ohio, the engineers explained.

Firestone is now spending heavily to overhaul its claims database and make it more usable for safety analyses, they said.

Firestone's sales of tires to consumers plunged 40 percent in September and October because of the furor over its products. The company says that its sales of tires to automakers have been little affected, although Ford has begun offering Michelin and Goodyear tires as alternatives to Firestones on Explorers. Bridgestone has set aside $750 million to cover the costs of the recall and lawsuits.

According to the engineers, the design problem lies in the so-called shoulder pockets of the recalled ATX tire. These are scalloped areas that run into the side of the tread from the sidewall to give traction in snow and during off-road driving. The steep angle of the ATX's shoulder pocket contributes to cracking inside the pocket, they said.

Particularly in tires made at the Decatur plant, the analysis found, the cracking was prone to spread from the pocket to another crucial part: the point where two belts of rubber-coated steel fibers form the tire's core.

Variations in the manufacturing process in Decatur meant that the thickness of rubber between the angle and the belts varied considerably, the engineers said — and sometimes was thinner than tire designers intended.

Decatur was also producing belts that did not stick to each other very well when vulcanized, a process in which a tire is heated and squeezed to fuse the various layers. Decatur's machinery for mixing rubber compounds produced pellets, the engineers said; the machinery at Firestone's other factories produced sheets of rubber.

In each case, the pellets or sheets would be sprayed with a lubricant to keep the rubber from sticking together in huge globs. According to the engineers, the pellets' greater surface area meant they were exposed to larger quantities of lubricant. When Firestone burned the rubber from the recalled tires' belts and measured the chemical properties of the ash, it found that 1 percent to 1.5 percent of the ash from tires made in Decatur consisted of lubricant — two to three times the amount found in ash from other tires. Extra lubricant makes the belts stick to each other less.

To address the finding, Firestone has begun shipping rubber sheets from its other plants to Decatur, where the pellets are being used only in less critical manufacturing processes, the engineers said.

Besides the design and manufacturing flaws, the Firestone investigation found that many tires involved in property damage and injury claims showed signs of customer abuse. These tires were twice as likely as the average recalled tire to have been repaired, usually for a puncture, and two-thirds of the repairs were done improperly, the engineers said. Punctured tires should have the hole plugged and the liner patched, but most often only one of these was done.

The tread separations would not have been so deadly if most of the Explorers had not rolled over when their tires failed, but Firestone has not analyzed the vehicle's stability, the engineers said.

A computer analysis earlier this fall by The New York Times of the federal government's database of all fatal crashes nationwide over the last nine years found that rollovers were a factor in 97 percent of tire- related deaths in Explorers.

Rollovers were a factor in 84 percent of the 377 tire-related deaths that occurred in all other sport utility vehicles during those years, but in only 38 percent of such deaths in cars.

One of the first big regulatory decisions facing the incoming Bush Administration will be whether to order a broader recall of Firestone tires. The National Highway Traffic Safety Administration has said that it will try to conclude its own review of the tires by March.