NHTSA also is investigating unintended acceleration in a dozen Toyota models, more than 8 million of which have been recalled globally since October.
And since November, NHTSA has been investigating reports of engine stalls in the 2006 Corolla.
Transportation Secretary Ray LaHood has been sensitive to questions raised in Congress about NHTSA's effectiveness. He is scheduled to testify Feb 10 before the House Oversight Committee, and House Energy Committee investigators have requested NHTSA documents dating to 2000.
After Toyota announced its hybrid recall, LaHood issued a statement that said: "When I spoke with Toyota President Akio Toyoda last week, he assured me that his company takes U.S. safety concerns very seriously. The U.S. DOT will remain in constant communication with Toyota to hold them to that promise."
Said Sean Kane, president of the Safety Research & Strategies consulting firm: "Toyota's Teflon image for reliability is going out the window faster than anyone can salvage. … People are logically asking, how can it have problems with so many models?"
Kane, who is scheduled to testify tomorrow at a congressional hearing on Toyota's problems, said that the accumulation of so many serious complaints over 21 months without an investigation raises questions about NHTSA's effectiveness.
Said Kane: "How could NHTSA's surveillance miss this apparent pattern?"
The Department of Transportation, which oversees NHTSA, said it receives 30,000 complaints a year, each of which it reviews within a single business day.
"NHTSA takes these very seriously and opens investigations based on the frequency and severity of the complaints," the e-mailed statement said.
NHTSA currently is conducting 40 investigations of possible defects, three of which involve Toyota, Transportation said. Over the last three years, NHTSA said its probes have resulted in 524 recalls involving 23.5 million vehicles.