"We heard an explosion - well, several explosions", shopper Christian Guibbert told reporters.
Then he went back into the supermarket's main room and walked towards the assailant, calling police and describing the situation.
"At that moment, he (the gunman) ran after me".
Police found the auto, and SWAT teams surrounded the market.
On Friday morning, an armed man broke into the store in the town of Trebes, firing shots and killing 2 people.
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When he went inside the supermarket, he left his phone on so police could hear his interactions with the gunman, Collomb said. "It was a heroic gesture and I'd rather we talk about him than the other one", she said of the attacker.
Changing target, he headed towards the riot police's garrison, where he shot at four officers jogging outside, before driving away.
The attacker was known to authorities for petty crimes but was not considered an Islamist threat, Collomb said, adding that he was believed to have acted alone.
The officer was shot and stabbed before anti-terror police moved in to kill the attacker and end the siege.
The violence unfolded when the attacker stole a auto, killing one person in the vehicle and gravely wounding another, said Collomb, the interior minister.
The incident revived memories of the January 2015 Hypercacher kosher supermarket siege in Paris, in which an ISIS sympathizer killed at least four hostages.
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He wounded an officer in the shoulder before heading to Trebes about 8km to the east, where two more people died.
President Emmanuel Macron said the terror threat in France "remains high" because of "radicalised individuals identified as risky who are being closely followed".
Philippe also said that one police officer had been injured but that his life was not in danger.
Macron rushed back from an European Union summit in Brussels to Paris, where counterterrorism investigators took over the investigation.
The president said he won't give details on the number of victims "at this stage".
"The monitoring. did not reveal any apparent signs that could lead (us) to foresee he would act", Molins said.
The gunman continued 6.5km farther east to the small town of Trèbes, with a population of 5,000.
He killed three people and wounded others before being shot dead when French police stormed the supermarket, authorities said.
Sixteen people were injured in the attack.
The spokesman said a police operation to apprehend the assailant is still underway.
Local authorities tweeted that the area was off-limits to the public.
Paris counterterrorism investigators took over the probe into the attack in Trebes and nearby Carcassonne.
"All the information we now have leads us to believe it is a terrorist act", Prime Minister Edouard Philippe said while on a visit to Mulhouse in eastern France, cutting short the trip.