Federal vehicle safety regulators Friday closed an investigation of two Tesla Model S sedan fires last year with a finding of no "defect trend," and the company offered to retrofit the electric cars with undercarriage plates to protect the battery.

The fires, in Tennessee and Washington, consumed two of the sleek sedans after they ran over roadway debris. Neither driver was injured.

Tesla said Friday that it is offering free retrofits to add a triple underbody shield to its Model S sedans at owners' requests. It said that all Model S sedans made since March 6 have been outfitted with the underbody shield.

The company's shares were up 2.4 percent at the close of trading after the announcement.

The National Highway Traffic Safety Administration said in closing its preliminary evaluation of the fires that the underbody retrofits should deal with the issue. But it reserved the right to take further action "if warranted by new circumstances."

The NHTSA said that "consumers should have their vehicles serviced promptly once they receive notification from Tesla Motors."

An analyst with Kelley Blue Book said the decision appeared to be essentially a recall without using that word, although Tesla denies that, saying it is making the fixes voluntarily.

"NHTSA's reaction to the problem and how they are resolving it is basically a recall," said Karl Brauer of Kelley Blue Book. "They are closing the investigation only because Tesla is making this change to cars they are producing and sending out notices, suggesting people come in and get the change to cars already made."

The decision was good news for Tesla, he said. "It could have been a much more expensive fix if NHTSA decided they needed to redesign the battery pack."

Elon Musk, Tesla's chairman and chief executive, blogged Friday that the modifications to the underbody bring the risk of fire from a collision with roadway debris "down to virtually zero."

Tesla earlier modified the vehicle's height with a software fix that increased clearance from the roadway, the NHTSA noted.

Though neither driver was injured in the two fires, the mishaps raised concerns that the battery was vulnerable to strikes from roadway debris.

In Tennessee, a sedan ran over a three-ball hitch that apparently fell from another vehicle and punctured the Model S battery.

Testing by Tesla showed the object could spear the battery with what it called a "piking effect."

A change in the vehicle's height mitigated that risk, the NHTSA said.

In the second fire, in Washington, the object that damaged the battery was not identified. That raised concerns about the effectiveness of merely increasing the height from the roadway, investigators said.

"Tesla's revision of vehicle ride height and addition of increased underbody protection should reduce both the frequency of underbody strikes and the resultant fire risk," the report said.

Musk blogged Friday that the retrofits should also prevent fire in "extremely high-speed impact that tears the wheels off the car."

That was the case in another Model S fire in Mexico when the car ran into a traffic circle at 110 mph and smashed through an 8-foot-tall concrete wall. The driver was uninjured.

Musk wrote that the underbody shields are a hollow aluminum bar designed to deflect objects, a titanium plate to protect front underbody components and a solid aluminum shield.

Contact Pete Carey at 408-920-5419. Follow him on Twitter.com/petecarey.