The investigation includes 15-inch Wilderness tires made in Wilson, N.C. and Joliette, Canada and all 16-inch Wilderness tires.

The only Wilderness tires Firestone agreed to recall last August were 15-inch tires made in Decatur, Illinois. The company also recalled all of its ATX tires.

Firestone has long insisted Decatur was the only plant that made bad Wilderness tires. But consumer activists dispute that claim.

"The Decatur plant is simply a subterfuge, a cover-up for attempting to avoid a broader recall. They want to place the blame on Decatur, what they should place the blame on is management and design of this tire and they should all be recalled," said Clarence Ditlow of the Center for Auto Safety.

The Center for Auto Safety is part of a class action suit that's attempting to use the courts to force Firestone to expand its recall, especially with the summer driving season right around the corner and temperatures rising to levels where the tires are most likely to peel to shreds.

"We're coming up on the most dangerous season for Firestone tire failures on Ford Explorers. That's the summer season when we have the hot weather, we have people driving long distances with heavy loads on. We're more likely to have emergency situations on the road and there's going to be an increased toll of deaths due to Firestone tires on Ford Explorers and we're not moving fast enough to correct it," cautioned Ditlow.

When told that Firestone says the problem has been corrected, Ditlow said, "No. Firestone has replaced some of the defective tires, not all."

Even an internal study conducted by Firestone seems to provide proof that people may still be driving on dangerous tires. In the study, Firestone engineers cut open and examined more than 400 undamaged Firestone tires to look for early signs of tread belt separation. When they specifically looked at samples from hot, southern states where the most damage is likely to occur, they found problems in 26 percent of the Decatur tires. But other plants were even worse... 48 percent from Wilson and 61 percent from Joliette.

Firestone says those findings were based on a small sample and not supported by other tests. But the tiremaker's own consultant, Sanjay Govindjee was at a loss to explain the discrepancy when asked about it in a recent deposition obtained by CBS News.

"Did that cause you to consider this may not be a Decatur only problem?" Govindjee was asked. "The results...I found rather counterintuitive..." he answered. "...I can't explain it."

"I'm convinced that more Firestone tires will be recalled, the only question today is who's gonna get there first: the government or the class action lawyers. And right now the class action lawyers are doing a better job than the National Highway Traffic Safety Administration which has already delayed a final decision twice," said Ditlow.

CBS News has learned that Firestone has been holding secret talks with consumer advocates and victim's attorneys discussing a possible settlement that sources say could include expanding the current recall.