RABAT,  Morocco — They called it a "solidarity caravan" — a group of Spanish volunteers delivering truckloads of donated computers, wheelchairs and other gifts for Africa's poor.

For the last eight years they had followed well-publicized routes through Morocco, Mauritania, Senegal and Gambia. This time, as the volunteers traversed the stretch of sand-swept pavement that is Mauritania's main highway, a group of armed men lay in wait.

Rather than solidarity, it seems, the gunmen sought hostages.

Members of the convoy heard gunshots at about 8 p.m. on Sunday, Nov. 29, they told reporters. Those in front raced back to the caravan's last car. The Land Rover stood abandoned in the dark, doors gaping open. Money and equipment remained inside. The passengers had vanished.

Spanish authorities have blamed the kidnapping on an organization   called "Al Qaeda in the Islamic Maghreb." It is the same group authorities believe abducted a French man from his hotel in northern Mali just a few days earlier.

The kidnappings are the latest evidence of the group's growing strength in a lawless region on the edge of the Sahara desert called the Sahel, which includes parts of the North African countries of Mauritania, Mali, Chad and Niger, security experts say.

"Terrorism is on the rise across the Sahel," said Justin Crump, a head of terrorism and country risk at the British security analysis firm Stirling Assynt. "There is little doubt that Al Qaeda in the Islamic Maghreb itself is extending its reach to the south."

Recent operations with clear links to the group have taken place in Mauritania, Mali and Niger, Crump said, but the organization first emerged in Algeria during that country's bloody civil war in the 1990s. Initially calling themselves the Salafist Group for Preaching and Combat, the Islamist guerillas targeted the Algerian government with the goal of establishing a Muslim state in the region.

But in 2003 they declared allegiance to Al Qaeda and its global goals. On Sept. 11, 2006, Al Qaeda's second in command, Ayman al-Zawahiri, approved the union in a video. Since then, the group has increasingly taken aim at foreign targets and expanded its membership beyond Algeria.

"Al Qaeda's role is to offer a compelling global jihadist vision, which has great appeal to religiously minded tribesmen," Crump said.

As the Algerian government has succeeded in limiting the organization's movements, the fighters have spread out to other countries in the region. "Al Qaeda in the Islamic Maghreb has found itself under increasing pressure in the north, where it used to be the most active," said Richard Barrett, who coordinates the United Nations Al Qaeda Taliban Monitoring Team. "And therefore it looks to its branches in the south to provide the logistical support, the financial support and everything else."

Kidnapping foreign hostages and releasing them for a ransom has become a lucrative business for Al Qaeda in the Islamic Maghreb. "You grab these people, you hold them for a bit and you find people pay up," Barrett said. "So you do it again."

In February 2008, members of the group kidnapped two Austrian tourists in Tunisia. Later that year, the terrorists abducted two Canadian diplomats who'd been traveling in Niger as part of a U.N. mission. This year, four European tourists were taken hostage in the border region between Mali and Niger. Al Qaeda in the Islamic Maghreb also claimed responsibility for killing an American aid worker in Mauritania's capital in what is thought to have been a botched kidnapping.

Mauritania, in particular, has seen an increasing number of violent attacks in the past several years. Four French tourists were shot while picnicking by the side of the road in 2007, in an attack the Maghreb Al Qaeda wing claimed as its own. The group has also claimed responsibility for two attacks on embassies in Mauritania's capital — French and Israeli — and for an assault on a military patrol that resulted in the beheading of twelve Mauritanian soldiers near the town of Zouerate.

Mauritania has a population the size of Chicago spread across a landscape larger than Texas and New Mexico combined. Among the Sahel states, Mauritania "is clearly one of the weakest and one of the easiest to operate in, in terms of vast uncontrollable desert areas," said Ruairi Patterson, a regional specialist for the British firm Control Risks. "It's got a very small population, never mind a very small army, so it's very difficult for the military to survey the entire terrain."

Crump said Al Qaeda's migration into Mauritania and Mali isn't just opportunism. He sees the hand of Al Qaeda's central leadership — all the way from its bases along the border between Afghanistan and Pakistan — pulling some 40 Mauritanian jihadists into the North African franchise.

"Although small in number, they bring stronger religious credentials to the group, which may help it to overcome some of its traditional internal rifts over tactics such as suicide bombing," Crump said.

The expansion comes even as the U.S. is funding counterterrorism efforts throughout the region. And it's not yet clear how well the operations are fighting insurgent activities.

"This is the great unknown," Crump said, "but the continued kidnappings and other largely unhindered operations are a sign that more needs to be done."