Six foreigners have been arrested as they flew into Mogadishu with $3.6m (£2.2m) in cash, Somali police say.
Three US citizens, two Kenyans and one Briton were detained in the capital, along with two aircraft, a spokesman told the BBC Somali service.
The spokesman said investigations were ongoing and did not give more details.
Unconfirmed reports say the money may have been a ransom payment for pirates who operate from Somalia, although they are mostly based in northern areas.
The AFP news agency quotes a security official as saying that one plane flew into Mogadishu, where the money was to be transferred to another aircraft which would deliver it elsewhere.
The Westerners arrested are understood to be white rather than of Somali origin.
Somalia has not had a strong national government for more than 20 years and money-laundering, piracy and militants groups have thrived in the country.
The UN-backed government is battling the al-Qaeda linked group al-Shabab in Mogadishu.
Al-Shabab and other militant groups control much of southern and central Somalia.
UN Security Council envoys are due to meet leaders of various Somali leaders in neighbouring Kenya later to discuss the country's future.
Donor countries are dismayed at the power-struggle between Somalia's President Sharif Sheikh Ahmed and speaker of parliament Hassan Sheikh Adam.
President Ahmed's term of office is due to expire in August.
He wants to extend it, while the speaker says MPs should hold elections for a new president.