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 .
A bomb went off on a bus in Guatemala City on Monday . At least six people were killed , including two minors . Bus drivers say they were warned of attacks if they did n't pay extortions . Comment: