At the Los Angeles Auto Show Tuesday, NHTSA Administrator David Strickland told CNBC there's no time frame for how long the investigation will take.
"We want to get this done right. Sometimes investigations take a handful of weeks, sometimes they take several months," said Strickland. "I have no real indication of how long this will take but with Tesla willing to be very proactive in helping us, hopefully it will be on the shorter end."
Strickland also said NHTSA decided independently to launch the Model S investigation and did not do so at the invitation of Tesla. That point counters the assertions of Tesla CEO Elon Musk who said his company asked NHTSA to open an investigation so it can clear up any questions about the safety of the Model S.
(Read more: Is Tesla the GM of tomorrow?)
Strickland said that once NHTSA contacted Tesla about its decision to open an investigation, the electric car company invited Federal investigators to do whatever is necessary as part of the probe.
Tesla stock still sliding
Since the first Model S battery fire just outside of Seattle on October 1, Tesla shares have dropped more than 30 percent and the company has lost over $9 billion in market value since hitting a high of $194.50 in September.
Despite the sell-off, Tesla shares are still up more than 250 percent in 2013.
—By CNBC's Phil LeBeau. Follow him on Twitter
Questions? Comments? BehindTheWheel@cnbc.com.