A bomb went off on a bus in Mexico City on Friday . 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 . At least six people were killed , including two minors . Bus drivers say they were afraid of death if they did n't pay extortions . Police are investigating all angles .
A bomb went off on a bus in Mexico City on Friday . At least six people were killed , including two minors . Bus drivers say they were afraid of death if they did n't pay extortions . Police are investigating all angles .