NOUAKCHOTT, Mauritania - Mauritanian prosecutors on Tuesday called the brutal slaying of four French tourists a terrorist act and said security forces were hunting three men suspected of links to a regional al-Qaida terror network.

Gunmen opened fire Monday on the tourists, who were members of the same family, while they were picnicking on the side of a road near Aleg, a small town 150 miles east of Mauritania's capital, Nouakchott, police said.

Though authorities initially thought the tourists had been robbed, subsequent investigations have shown they were not, the Interior Ministry said in a statement Tuesday.

The sole survivor, the family's father, was seriously injured and flown to neighboring Senegal where he was hospitalized in intensive care in the capital, Dakar, said Moussa Samb, spokesman for the hospital. French radio station France Info said the father would be sent Wednesday to France and hospitalized in Lyon.

Police and French diplomats in Mauritania say the surviving man is in his 70s.

A statement issued by the public prosecutor's office in Mauritania's capital said the attack was perpetrated by three men who it said were known members of the Algeria-based terror network al-Qaida in Islamic North Africa.

Police believe the three fled toward Senegal after the attack and may have crossed the border into that country. Senegalese authorities contacted in the region said they were aware of the case and were hunting for the men, but could not confirm whether they were in Senegal.

"This was a grave terrorist act committed by dangerous criminal terrorists," judge Moustapha Ould Said told The Associated Press.

At least two of the suspects had been accused by prosecutors earlier this year of being members of the al-Qaida-linked terror group and of having undergone military training with them in Algeria.

The statement said the trial of one of the men earlier this year was postponed. The second man was acquitted, but prosecutors appealed his case. There were no details on the third suspect.

Police said Tuesday they detained a man who allegedly arranged a taxi for the three men suspected in the attack.

The man who allegedly arranged the taxi was tried earlier this year on charges similar to those against the suspected gunmen. He was convicted of having trained with the al-Qaida group in Algeria and given a two-year suspended sentence.

Three other common criminals detained Monday by authorities in connection with the slaying of the French tourists were released Tuesday.

Such violence is rare in Mauritania, a relatively stable democracy in northwestern Africa perched on the far western rim of the Sahara desert.

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Associated Press Writer Jenny Barchfield in Paris contributed to this report.