The Pentagon says it will open a formal investigation into what Syrian monitoring groups say is the U.S.’s deadliest attack on civilians since the U.S. began bombing Syria nearly two years ago. The strike took place 10 days ago near the city of Manbij. The monitoring group Airwars said the strike killed between 120 and 150 civilians. Until earlier this year, the Pentagon claimed no civilians had been killed in its air campaign against ISIS, while at the same time claiming more than 20,000 ISIS fighters had been killed.