Last year, owners of about 16.2 million cars and trucks received letters saying their vehicles had safety problems and were being recalled. But millions more were left in recall limbo because some of the National Highway Traffic Safety Administration's investigations into possible safety problems have been taking years longer than the agency's self-imposed goal.
Consequently, some owners may be unknowingly driving vehicles with problems, said Joan Claybrook, the onetime agency administrator and former head of Public Citizen, a consumer protection group.
Some consumer advocates say the agency needs more investigators.
But that's not true, said David Strickland, the agency's current administrator. "Clearly we are able to succeed in executing our mission and doing it very well with the staff on hand and the expertise that we have on hand," he said in a telephone interview.
Among the consumers Ms. Claybrook worries about are 1.5 million owners of 2002-5 Ford Explorers and Mercury Mountaineers and about 1.7 million owners of 1999-2003 General Motors full-size pickups and sport utility vehicles.
The Ford Explorer-Mountaineer investigation went on for 42 months, and the G.M. truck inquiry continues after 25 months. The agency's self-imposed goal is to complete such investigations, called engineering analyses, in 12 months.
Often manufacturers learn of a safety problem on their own and, as required by law, inform the agency of their plans for a recall. Other times, owners complain to the agency, prompting an investigation.
Those inquiries are the responsibility of a few more than two dozen agency investigators, a number that hasn't changed much over more than a decade. In 2001, the agency had 26. But despite the growing complexity of vehicles, there are currently 28, the agency says.
That is less than one investigator for each automaker, truck maker or parts supplier, according to Clarence Ditlow, executive director of the Center for Auto Safety. "The enforcement arm of the agency is on a starvation diet," he said.
One of the agency's continuing problems has been finding money to hire enough investigators, said Michael Brownlee, who was associate administrator for enforcement when he retired in 1997.
"It was always a fight for dollars," Mr. Brownlee said. "I mean, that is the bureaucratic nightmare every year." But the current administrator, Mr. Strickland, says there are enough investigators in part because new tools for data analysis allow them to work more efficiently. Mr. Strickland noted that agency investigations resulted in 134 vehicle recalls in 2012, the second-highest number since the government began counting recalls in 1966.
That resulted in about nine million vehicles being recalled.
"Clearly we are able to succeed in doing our mission," he said.
But the number of recalls is an imperfect measure of productivity because the agency is using an accounting method under which one inquiry can generate dozens of recalls.
For example, investigations resulted in 131 recalls in 2011. But 61 of those resulted from one investigation involving Webasto aftermarket sunroofs that might loosen and fly off the vehicle. Any car dealership or business that installed one of the sunroofs was listed as a separate recall. In 2012 -- the year cited by Mr. Strickland -- there were 19 sunroof recalls.
An investigation into a brake actuator used on trailers resulted in 34 recalls in 2012.
Mr. Ditlow, of the Center for Auto Safety, said: "The historic rule at N.H.T.S.A. is one investigation, one recall. Anything else is recall inflation."
In the current budget, Congress gave N.H.T.S.A. about $800 million, which allows for about 600 employees. About $10.6 million of that goes to the Office of Defect Investigations, and about $554 million goes to states for their highway safety programs.
Over the years, appropriations for such state traffic safety programs have increased, but money for defect investigations hasn't kept up, said Jacqueline S. Gillan, president of Advocates for Highway and Auto Safety. a group financed by insurance companies. There should be more money for investigations, she added.
Concerns about how defect investigations are carried out is an old issue. For about three decades the Government Accountability Office has sporadically scolded the agency, saying it needs to improve the recall process and reduce the length of investigations. Agency officials have responded that they have been addressing concerns and improving.
An October 2011 report by the Transportation Department's inspector general noted that not all investigations were completed in a timely manner. The report said a review of 20 engineering analyses found 40 percent missed the deadline by an average of six months. The worst case was 17 months.
But the agency has been far tardier.
In August 2009, it opened an investigation over complaints from 200 owners of 2002-5 Ford Explorer and Mercury Mountaineer S.U.V.'s who said the vehicles moved even if the automatic transmission was in Park. The complaints included 85 "alleged crashes and 32 alleged injuries."
The inquiry continued for 42 months, two and a half years beyond the agency's 12-month goal. It was officially concluded -- without a recall -- shortly after a reporter asked an agency official about it. The closing report said there "were very few incidents occurring in recent years."
Another continuing engineering analysis involves about 1.7 million 1999-2003 full-size pickups, including the popular Chevrolet Silverado, and S.U.V.'s including the Suburban.
The concern is that brake lines are so susceptible to rust they may burst, making the vehicles harder to stop.
That investigation was prompted by Robert Slamer of Middletown, Ohio. Early in 2010, he asked for an investigation after a brake line burst on his 2003 Silverado 2500 heavy-duty pickup -- with about 51,000 miles on the odometer -- while his wife, Martha, was hauling a load of hay. Mr. Slamer says it took a long time to stop even through the vehicle was traveling slowly.
The agency granted Mr. Slamer's request in January 2011 and opened an engineering analysis. In researching the issue, the agency found 519 complaints, including 13 crashes and two injuries. That investigation is not complete.
In a telephone interview, Mr. Slamer said he was worried that someone would be injured or killed and that he was amazed the investigation was still going on after two years.
Mr. Strickland said that every investigation was different, that some were far more complicated than others, that accuracy was crucial and that the 12-month goal was self-imposed.
"The most important thing is for us to get it right," he said.
A review of agency records shows that six of the 15 engineering analyses begun in 2011 are still under way.
During such long investigations some consumers are left waiting. One of those is Julie Miller of Hopewell Junction, N.Y., who owns a 2004 Ford Freestar.
Last April, the agency opened an engineering analysis into reports that corrosion in the minivan's rear wheel wells was so severe that the third row of seats might not be solidly anchored.
Ms. Miller says she is afraid to use the Freestar to carry her family of five and doesn't know what to do about getting it fixed.
She says a welder told her he could fix the back seat for $800. A body shop that works with a Ford dealership gave her an estimate of $11,000, apparently to replace the wheel wells.
She worries that if she opts for the $800 repair and Ford agrees to a recall, the automaker may tell her that the fix was not done in the recommended manner and will refuse to reimburse her.
"I honestly don't know what we are going to do," she said.
This article originally appeared in The New York Times.