The WSJ had the most detailed coverage of the killing. The key paragraphs from Nicolas Casey’s story:
According to two witnesses on the bridge, the victim [Sergio Adrián Hernández] was part of a group of teens who had sidestepped border checkpoints on the Santa Fe Bridge—which is flanked by border checkpoints on either side—and entered the U.S. on foot, crossing a dry aqueduct and an old railroad beside the bridge. None were carrying backpacks or appeared to have weapons, the two witnesses said.
The teens were playing a kind of “cat-and-mouse game,” said Bobbie McDow, 52, a U.S. national who said she witnessed the shooting from the middle of the bridge where she was standing. The teenagers, Ms. McDow said, appeared to be trying to make it to the U.S. side and quickly back to Mexico without being caught by officials, a pattern that Ms. McDow said she has noticed.
Ms. McDow said that U.S. border agents spotted the teenagers and gave chase. One teen stumbled and was caught, and another was pinned down by an agent who had chased him by bicycle, she said. While holding down one of the Mexican boys, this agent fired shots toward Mexico.
One of the youths—not the young Mr. Hernández—had thrown rocks at the border patrol agents, Ms. McDow said, but she stressed that the agent’s “life wasn’t under threat.” (WSJ 6/8)