The Source spoke with Sergeant Jerry Johnson at the Alhambra Police Department, who said that the APD followed up with various electric, public works and transportation agencies in the area, and ruled out causes from them. "We're not sure what these booms are, but it's more likely that they're fireworks," he said, adding that the department would continue to investigate any subsequent reports.
Furthermore, excluding 4th of July, the police department has received 12 to 15 reports of these "unexplained booming noises."
Caltech seismologist Jennifer Andrews, who was asked to check seismic activity in the area on the dates of these noise reports, said that she still hasn't detected any unusual activity.
Police departments in surrounding communities have not received any similar reports.