NEW YORK (

TheStreet

) -- U.S. safety regulators are investigating

Toyota's

(TM) - Get Report

electronic throttle system as a possible cause of its vehicle sudden acceleration problems -- which could spell that more serious problems lie ahead for the automaker.

Toyota has blamed the sudden acceleration problem on gas pedals getting trapped in floor mats as well as "sticky gas pedals" connected to a vehicle friction device, but investigators believe the problem could be much graver than that.

Regulators say the problem, which has been reported over the last ten years, could be connected to an electronic throttle system in Toyota and Lexus vehicles; and if that were indeed the case, the problem would be much more difficult and expensive for Toyota to fix. It would, of course, also add to the view that Toyota hasn't been completely honest about the acceleration problem and cause further distrust between the automaker and its customers.

The scrutiny over the electronic throttle issues have intensified as congressional leaders and safety investigators proceed with new probes of Toyota over the auto-malfunction amid its mass vehicle recalls.

While Toyota executives have presented a clear explanation of the sudden acceleration of the problem to the public, reports say that they have acknowledged some uncertainty about the root cause of the problem behind the scenes.

On February 1, Jim Lentz, the president of Toyota USA, said he wanted to "sincerely apologize to Toyota owners" over the recalls and auto-malfunction. Lentz said in a statement that many Toyota dealers would be extending their hours -- and some will remain open 24 hours -- to fix the problem.

"And we're redoubling our efforts to ensure this does not happen again," Lentz said.

Toyota is reportedly now under the threat of possible civil penalties for its handling of the recent recall; the company has been widely criticized for being too slow to react and remedy the rapid acceleration problem that has been linked to fatalities.

Toyota stock has tumbled 3.4% to $75.50 Wednesday on the news.

Meanwhile,

rival companies' stocks are performing better.

Ford

(F) - Get Report

stock has risen 0.6% to $11.50, while

Honda

(HMC) - Get Report

has surged 4.1% to $35.70.

Daimler

( DAI) stock is up 2.4% at $48.40.

-- Reported by Andrea Tse in New York

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>> Can Ford Reclaim Toyota's Throne?

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