Kenyan police said on Friday they had arrested two Iranians after they seized chemicals they suspected were going to be used to make explosives in Mombasa, which has been hit by a series of attacks.

The port city, the capital Nairobi and other parts of Kenya have suffered a series of grenade attacks since Kenya sent troops into Somalia last year to try to crush al Shabaab insurgents it blames for a surge in violence and kidnappings threatening tourism in east Africa’s biggest economy.

Police arrested the Iranians on Wednesday in Nairobi. On the same day, police impounded a container in Mombasa originating from Iraq and suspected to be carrying explosives.

On Thursday, police flew one of the suspects to Mombasa, where he led police to recover 15 kg of powder, which security experts took to their laboratory for testing.

“They are cooperating well. They are giving us key information that might help us reduce terrorist attacks in the country,” Ambrose Munyasia, a senior police officer at the Coast region told Reuters.

“We want to find out whether these substances are linked to any terror groups, including al Shabaab, al Qaeda and any other group,” Aggrey Adoli, Coast provincial police officer, added.

Francis Kimemia, Kenya’s acting head of civil service who was in Mombasa, said the government had sought the help of international agencies such the FBI and Interpol in helping deal with security threats.

“We have been working with them in terms of identifying criminals. We cannot fight terrorism alone. You have to work with other partners and other state organs,” he said.

In the most recent attack, a bomb exploded in a trading center in the heart of Nairobi in late May, wounding more than 30 people. One person later died from their injuries.

Gunmen also detonated grenades outside a nightclub in Mombasa in May, killing one person and wounding several others.

Al Shabaab seeks to impose a strict version of sharia, Islamic law. The group emerged as a force in 2006 as part of a movement that pushed U.S.-backed warlords out of Somalia’s capital, Mogadishu.

At present it also has hundreds of foreign fighters in its ranks.