Burkina Faso police were questioning several people on Wednesday, including four seen talking to the jihadist gunmen who killed 30 people after storming a top hotel and a cafe in the capital, a security source said.
Two of the 30 dead, around half of them foreigners, have still not been identified, "one black person and one white," prosecutor Maiza Sereme told AFP.
She said French and US investigators were working with local police on the probe, as well as experts from Canada and police from Niger.
A video recovered in the four-star Hotel Splendid targeted in Friday's attack in Ouagadougou shows "four people communicating with the jihadists. We identified and found them. They are being interrogated," a security source told AFP on condition of anonymity.
Another security source added that police had detained four Niger nationals as well as more than a dozen members of a Tuareg rebel group, the National Movement for the Liberation of Azawad (MNLA).
Meanwhile Italy's Mario Giro, the under-secretary for foreign affairs, laid a wreath at the Cappuccino cafe near the Splendid hotel where a nine-year-old Italian boy was killed along with 25 other victims of the attack.
"We must not forget, so this never happens again," Giro said.
"Burkina was hit because it's an example of democracy in this region and in Africa and because it's traditionally been a place where Christians and Muslims live together."
Among the four Niger citizens detained was a contender for next month's presidential elections, Adal Rhoubed, a doctor of Tuareg origin who runs a clinic in Tahoua in western Niger and who supporters said travelled to Burkina as part of his electoral campaign.
The security source who spoke to AFP said the Niger nationals were pulled in for questioning "to check their identities and schedule."
DETAINED FOR QUESTIONING
Some 15 members of the MNLA were detained for questioning in different parts of Burkina Faso, "some then freed and others not."
Police were also reported to be making arrests in a refugee camp sheltering Malians in Mentao, near the town of Djibo, where an elderly Australian couple were kidnapped at the weekend.
Residents of the remote, rural community have opened a Facebook page calling for the release of "the doctor of the poor", Dr Ken Elliot, and his wife Jocelyn, on the night of January 15-16.
The elderly couple from Perth spent some 40 years running a 120-bed clinic there, the only medical facility in the region.
Earlier this week, hundreds of students in khaki uniforms with hand-printed cardboard placards reading "Free Elliot" turned out in Djibo with their teachers.
Separately, French Defence Minister Jean-Yves Le Drian on Wednesday said recent jihadist attacks in Burkina and neighbouring Mali had only served to bolster the resolve of France's counter-terrorism mission in the Sahel region, known as Operation Barkhane.
The attacks "have shown that our work in the Sahel is not done yet," Le Drian told reporters in Paris.
"I've always know it would be a long-term job," he said, adding that France was satisfied with Operation Barkhane and had no plans to change strategies.