Prosecutors in Mauritania have alleged that men suspected of murdering four French tourists are linked to a group allied to al-Qaeda.
The survivor of the attack has now been transferred to Senegal
At least two of three suspects now being hunted by authorities were linked to the group by prosecutors.
Four tourists - apparently members of the same family - were shot by gunmen near the town of Aleg on Monday.
A fifth person, said to be the father of the family, survived and has been transferred to Senegal for treatment.
Some arrests have been reported, though the number of arrests and the identity of those detained is unclear.
Such an attack is rare in the south of Mauritania, a former French colony which enjoys relative stability. The north of the country is generally regarded as less safe for travellers.
Tuesday's attack happened after the five victims stopped on the side of a road for a picnic.
The gunmen opened fire with automatic weapons, before speeding off in a car that was later recovered in Aleg, about 250km (160 miles) east of the capital Nouakchott.
The victims were initially reported to have been robbed, though the interior ministry now says they were not, the Associated Press news agency reports.
In their statement, prosecutors in Nouakchott said murder suspects were thought to belong to al-Qaeda in the Islamic Maghreb - a group formerly known as the Salafist Group for Preaching and Combat.
However, reports differ on whether prosecutors believe two or all three of the men currently being sought belong to the group.
Hunt for killers
The three suspects are said to have fled south towards the border with Senegal following Tuesday's attack.
On Monday, French President Nicolas Sarkozy spoke to his Mauritanian counterpart, who assured him everything was being done to find the killers.
Mr Sarkozy later told reporters details of the incident were unclear.
"As things stand, I can only confirm this sad news and offer my condolences," he said.
Mauritania is one of the world's poorest countries, but has hopes for future prosperity based on the exploitation of its offshore reserves of oil and natural gas.