The military on Friday ordered soldiers in a section of southern France to wear civilian clothes when off-base for extra security following deadly, unsolved shootings of paratroopers in the area.
Police brought in counterterrorism experts and specialists in serial killers as they pieced together clues over the shooting deaths Thursday of two paratroopers, officials said.
At a news conference Friday, Toulouse state prosecutor Michel Valet said the gun fired was the same one used in a deadly shooting of a soldier four days before.
He said authorities weren't ruling out any theories about what was behind the attacks, but said a settling of scores was possible and there were no signs so far suggesting terrorism was involved.
As a precaution, the army called for enhanced vigilance among the 7,000-8,000 troops in the 11th paratroopers brigade based in and near Toulouse, and ordered them to dress in civilian attire when off-base, said Lt. Col. Pierre-Yves Sarzaud, an army spokesman.
A gunman on a motorbike opened fire on three uniformed paratroopers at a bank machine Thursday in the southern town of Montauban, killing two and critically wounding the other. The attack occurred not far from their barracks. The gunman appeared to be waiting for them, police said.
Four days earlier, a gunman on a motorbike shot and killed another paratrooper in Toulouse, about 50 kilometers (30 miles) away.
Valet said forensic analysis showed that the same weapon was used in the shootings in Montauban and Toulouse.
A judicial official said authorities are examining several possible theories as they try to find the attacker and motive for the shootings.
Both officials were not authorized to be named because the investigation is ongoing.
French media percolated with speculation about what was behind the attacks. Such incidents are rare in France, where gun controls are relatively tight.
The three men targeted Thursday served in a regiment that lost four soldiers in fighting in Afghanistan, the 17th paratrooper combat engineering regiment, Montauban Mayor Brigitte Bareges said.
However, Thursday's victims had not served in Afghanistan, according to a military official.
One of those killed had served in Africa, while the other two had served in French territories overseas but not in other countries, the official said.
France has some 7,250 troops deployed in foreign operations _ about half of them in Afghanistan _ and most of the rest in Africa and on a U.N. peacekeeping mission in Lebanon, according to the most recent Defense Ministry figures on its website.
Autopsies were under way Friday for the two soldiers killed in Montauban, a 25-year-old and 23-year-old who were both hit in the head in a 13-shot barrage of gunfire _ some fired point-blank, Valet said. The girlfriend of the younger victim was seven months pregnant.
The third soldier, aged 27, remained in critical but stable condition, Valet said. Their identities were not released.
The national judicial police are overseeing the investigation, a service that includes anti-terrorist investigators and specialists in serial crimes, the judicial official said.
Defense Minister Gerard Longuet said he was heading to the site Friday.
Associated Press writers Angela Charlton and Jamey Keaten in Paris contributed to this report.