As the U.S. Congress gears up to delve into Toyota Motor Corp's <7203.T> massive global vehicle recall, the question they face is whether the automaker and regulators misread or ignored rising consumer complaints.

Complaints about unintended acceleration rose sharply from model years 2002 to 2007 -- a period when Toyota expanded its use of electronic throttle controls, according to an analysis of data compiled by Safety Research and Strategies.

But by 2007, only two formal federal investigations had been launched with one leading to a modest recall -- for floor mats. Some 1,700 complaints have been raised for the six model years, peaking at 400 for 2007.

About one-third of those sudden acceleration complaints involve Toyota's best selling Camry sedan.

Potentially more worrying for regulators and consumers: More than half of the 2,262 complaints compiled by Safety Research involve vehicles that Toyota has not recalled.

Toyota has recalled more than 8 million vehicles worldwide for problems with either floor mats , which can trap a gas pedal, or a mechanical glitch in the accelerator, which can cause the pedal to become stuck.

But safety advocates and lawyers for a Michigan woman killed in a high-speed wreck in April 2008 argue that evidence suggests electronic throttle controls are at fault, not floor mats or sticky pedals.

The National Highway Traffic Safety Administration said in early February it would review the past probes that found no problems with electronic throttle systems even though complaints had grown steadily during times of those reviews.

What appears crucial is the role of Toyota and U.S. regulators in narrowing the focus of earlier safety investigations.

In one case Chris Santucci, a former U.S. safety regulator hired by Toyota in 2003, played a role in discussions with NHTSA during a 2004 probe in which the agency sharply narrowed the scope of its inquiry, according to his deposition in a lawsuit seeking damages from the Michigan crash.

The substance of the discussions between the Toyota representatives and NHTSA investigators was unclear.

In that instance, NHTSA agreed to exclude from consideration reports of uncontrolled acceleration in Toyota vehicles where the incident lasted more than a few seconds or the driver hit the brakes.

As a result, Toyota did not provide information to the U.S. government about reports it had taken about such cases.

Under the narrower scope of the investigation, NHTSA eliminated nearly all of the 260 complaints it received and closed the investigation without taking action in July 2004.

Toyota defended Santucci.

Mr. Santucci has an exemplary professional reputation that he earned by working diligently on safety issues at NHTSA as well as here at Toyota, Toyota spokeswoman Martha Voss said in a statement.

Industry professionals across the spectrum who know Mr. Santucci would agree that any insinuation that he violated federal ethics laws or that he did not live up to the highest professional standards is reckless and without merit.

Critics say NHTSA has a number of tools that have gone unused. Supporters say regulators are overwhelmed by 30,000 complaints per year and a mandate that forces them to confront deep-pocketed automakers loaded with engineers, technical analysts, lawyers and political muscle.

Two U.S. House of Representatives committees want to examine the government's response to the Toyota recalls and get a better understanding of what is behind unintended acceleration.

Transportation Secretary Ray LaHood, NHTSA Administrator David Strickland and Toyota's top North American executive Yoshi Inaba are due to testify at one hearing on February 24. The second is also expected to draw a heavyweight witness list.

In a statement published in the Washington Post on Tuesday, Toyota President Akio Toyoda said he had personally assured LaHood that lines of communications with safety agencies and regulators will be kept open ... and that we will be more vigilant in responding to those officials on all matters.

Toyota began using electronic throttle controls, or a drive-by-wire system, in U.S.-sold vehicles with 1998 models and expanded their use with 2002 models.

Complaints of sudden acceleration soared more than tenfold when Toyota switched to an electronic throttle for the 2002 model year Camry, its best-selling vehicle, according to data compiled by Safety Research.

The founder of the firm, Sean Kane, has been called as an expert witness in one of the congressional hearings.

The spike in complaints about Toyota vehicles coincided with a growth spurt for the automaker, which roughly doubled its U.S. market share and saw sales leap nearly 80 percent from 1999 to 2006.

Safety Research puts the number of unintended acceleration cases at over 2,200, including 26 reported deaths.

Toyota has employed several strategies to deflect the agency investigations, Kane said in a report on Friday.

The automaker declined to comment on Kane's report.

LaHood has said NHTSA has the tools to do a thorough review of safety issues related to the electronic controls on Toyota cars.

(Reporting by Steve Gorman, John Crawley, Bernie Woodall, Kevin Krolicki, Nick Carey, David Bailey and Soyoung Kim; editing by Andre Grenon)