The Japanese partmaker could be backed up for months with requests for replacement air bags. (Toru Yamanaka/AFP/Getty Images)

Nearly 34 million cars and trucks nationwide were declared defective Tuesday because of deadly air bags made by auto-parts giant Takata, in what is expected to be the biggest recall of any consumer product in U.S. history.

The expanded recall doubled the number of vehicles believed to have the air bags, which can blast out sharp metal shrapnel when deployed, a flaw that has been linked to six deaths and more than 100 injuries.

The nationwide recall effort is expected to be a logistical nightmare for the auto industry, costing billions of dollars and potentially overwhelming automakers, parts suppliers and dealerships already struggling to find enough safe replacement parts.

It could be days before vehicle owners hear from automakers about whether their models are covered by the recall, officials said, and analysts expect that it could take years for all the defective cars to get the needed repairs.

In the meantime, millions who drive some of the most popular models from BMW, Ford, Honda, Toyota and other carmakers could remain behind the wheel with a defect that lawmakers have deemed a “public safety threat.”

In these two forensic tests, shrapnel flies out of a Takata air bag when crash conditions are simulated in a test environment. The tests, conducted in December 2014, were commissioned by plaintiff's attorney Kevin Dean, whose firm, Motley Rice LLC, is representing several plaintiffs involved in Takata air bag litigation. The Post has edited this video to highlight the shrapnel. (Cameron Blake/The Washington Post)

[READ: Is your car part of the recall?]

“How long is this going to take? Nobody knows that yet,” National Highway Traffic Safety Administration chief Mark Rosekind said Tuesday, referring to the recall process.

Transportation Secretary Anthony Foxx added that it was “probably the most complex consumer safety recall in U.S. history.”

The Takata recall eclipses General Motors’ recall of 30 million vehicles last year for faulty ignition switches and other problems. It also surpassed the biggest previous consumer recall, in 1982, of 31 million bottles of Tylenol amid a poison scare.

Federal officials said years of humid weather, along with other factors, could cause the propellant in driver- and passenger-side air-bag inflaters to burn hotter than it should, leading to shard-blasting ruptures that Takata blamed on “over-aggressive combustion.”

But an investigation by Takata, automakers and independent researchers has yet to point to a definitive cause behind the ruptures, leading safety advocates to worry that replacement parts could have the same fatal flaw.

Takata, a Tokyo-based parts manufacturer, supplied an estimated 30 percent of the world’s air bags to several global automakers, and its defective parts have been found in dozens of car and truck models made since 2000.

Japanese auto-parts manufacturer Takata has agreed to declare nearly 34 million vehicles defective due to problems with air bag inflators. The recall will be the largest of any consumer product in U.S. history. (Reuters)

Federal officials said they did not yet know exactly which makes and models were covered in the recall. Nearly a dozen automakers, including Honda, Toyota and Ford, have already recalled 17 million potentially defective vehicles across the United States and more than 36 million worldwide.

But Takata bitterly resisted an expanded recall for months, opting to limit the effort to humid regions where high moisture levels worsened the defect. Early recalls, Takata said, will target older cars in more humid climates before expanding across the country.

The expanded recall is a victory for NHTSA, which fought with Takata for months and was criticized by lawmakers last year for its public guidance amid the regional recall. Foxx called it “a major step forward for public safety,” adding: “We will not stop our work until every air bag is replaced.”

“It’s a huge, huge recall, and it’s really a relief to know that this has now been resolved,” said Joan Claybrook, a former NHTSA head. “This is such a dangerous system that I don’t think anyone is going to hesitate to bring their car in to get it fixed.”

Shigehisa Takada, Takata’s chairman and chief executive, said Tuesday that the company “was pleased to have reached this agreement with NHTSA, which presents a clear path forward to advancing safety and restoring the trust of automakers and the driving public.”

In February, NHTSA began fining Takata $14,000 a day for failing to cooperate with its investigation. That fine, which has reached about $1.2 million, was suspended Tuesday following the expanded recall.

Some lawmakers celebrated the recall as a step toward safer roads. Sen. Bill Nelson (D-Fla.), a persistent Takata critic, said, “Folks shouldn’t have to drive around wondering if their air bag is going to explode in their face.”

But others worried that more questions needed to be answered. In a statement, Democratic Reps. Frank Pallone Jr. (N.J.) and Jan Schakowsky (Ill.) urged Takata and NHTSA to intensify their investigation, adding that they were concerned that “the root cause of these deadly deployments is still unknown.”

Replacing the defective parts could be a worldwide challenge. Takata, which runs 56 plants in 20 countries and employs 36,000 workers worldwide, has said it could make millions of new air bags a year, but not tens of millions.

Automakers such as Honda, believed to be the worst affected, have signed deals with other suppliers and manufacturers, including Autoliv and TRW Automotive, for replacement parts. Even so, some automakers were telling drivers that it could be months before their cars are repaired.

“A recall of this scope illustrates the potential for massive automaker expense and consumer inconvenience when a common, mass-produced part is defective,” said Karl Brauer, a senior analyst at Kelley Blue Book. “Ironically, the use of common parts across markets and manufacturers is meant to save money, yet a recall of this size will cost the industry billions.”

On top of its standard first-class recall mailers and more than 1 million phone calls to owners, Honda has told its dealerships to check vehicles’ identification numbers during every visit and to offer free rental cars during repairs, Bruce Smith, American Honda Motor’s senior vice president, said last month at a NHTSA recall workshop.

Honda workers have been scouring salvage yards, collecting more than 20,000 defective inflaters to prevent them from being used in repairs, Smith said. The carmaker has also enlisted an investigative firm in hopes of finding hard-to-reach owners of older vehicles with the recalled part.

The Justice Department is investigating Takata, and numerous lawsuits have slammed the parts maker’s handling of the recall.

Orlando lawyer Richard New­some represents seven clients who say they were injured by Takata air bags. One Florida man, Corey Burdick, said he was blinded in one eye last year after the air bag in his 2001 Honda Civic deployed, sending a metal projectile into his face.

“We still don’t know how this is happening,” Newsome said. “Are the replacement air bags going to have the same problem?”

More than 50 million cars and trucks were recalled in 2014 in the worst year for recalls in U.S. history, with 1 in 5 vehicles on the road said to be at risk of critical defects.

Drivers can find out whether their cars are included in a recall by visiting vinrcl.safercar.gov/vin. Federal officials have also launched a “recalls spotlight” section of Safercar.gov to provide updates.

Clarence Ditlow, executive director of the Center for Auto Safety in Washington, said the millions of vehicle owners who have already received recall notices should set up appointments with their dealerships soon, since ­automakers believe they’re at greatest risk.

“No matter how you look at it, it’s going to be a mess,” Ditlow said. “The real question: Is this recall going to be big enough?”

Todd C. Frankel contributed to this report.