Guatemala City, Guatemala (CNN) -- Six people were killed and 17 were injured in a bomb blast onboard a bus in northwestern Guatemala City, officials said.
Police on Tuesday were still not sure who set off the bomb or why, though one transportation union leader said that gang members had threatened violence if bus companies did not pay extortions.
Initial reports from police on Monday were of four or five people dead, but the number rose to six Tuesday morning, CNN affiliate Noti 7 reported.
The injured remained at the hospital, where they were being treated for second- and third-degree burns.
A mother and her two children of 11 and 13 years were among those killed, local media reported.
Some witnesses told police that a woman came aboard the bus, placed the bag that presumably held the explosive on the luggage rack, and then got off the bus, Noti 7 reported. But authorities added that they are investigating all possibilities.
Gamaliel Chan of the bus drivers' union said that his organization had warned authorities of the possibility of an attack because of threats made to them by gang members. The gang members allegedly told the drivers that there would be an attack if they did not pay a Christmas "bonus."
According to Noti 7, the gang members were asking for 60,000 quetzals from the bus drivers, or about $7,300.
Journalist Alexia Rios contributed to this report.