The gang is notorious for kidnapping groups of people for ransom.
The same gang, 400 Mazowo, abducted a group of Catholic clergy in April.
The clergy were later released but it is not clear if a ransom was paid.
Who are the victims?
All of those kidnapped are US citizens, except one who is a Canadian national.
Among those seized are five men, seven women and five children. The youngest child is reportedly only two years old.
They worked for Christian Aids Ministries, a non-profit missionary organisation based in the US state of Ohio, which supplies Haitian children with shelter, food and clothing.
How were they kidnapped?
The missionaries were returning from a visit to an orphanage when the bus they were travelling in was seized by the gang members on a main road in the town of Ganthier, east of the capital, Port-au-Prince.
Ganthier is located in the Croix-des-Bouquets area which is controlled by 400 Mazowo gang.
Seizing vehicles and all of their occupants for ransom is one of the main activities the 400 Mazowo gang uses to finance itself.
The Washington Post said one of those abducted had posted a WhatsApp message calling for help.
"Please pray for us!! We are being held hostage, they kidnapped our driver. Pray pray pray. We don't know where they're taking us," it said.
What's the reaction been?
The White House said on Monday that both the US Department of State and the FBI were assisting Haitian authorities with the case.
A former field director for Christian Aid Ministries in Haiti told CNN that the kidnappers had already made contact with the missionary organisation.
Adam Kinzinger, a Republican congressman from Illinois, told CNN he believed the US should negotiate with the kidnappers, but not pay ransom.
"We need to track down where they are and see if negotiations - without paying ransom - are possible," he said. "Or do whatever we need to do, on a military front or police front."
In Haiti, local unions staged a walkout on Monday in protest at the rising levels of crime.
The strike closed down business in Port-au-Prince and other cities, as public transportation employees stayed home. Barricades were set up in some areas to prevent workers from crossing picket lines.