Federal investigators saw videos eight months ago of a U.S. Border Patrol Agent punching a teenager in the stomach, but until now, they allowed the agent to continue working unpunished.

That agent, 10-year Border Patrol veteran Aldo Arteaga, now faces assault charges in Arizona over the gut punch of the detained teenager, The Associated Press reported.

Arteaga was initially charged last week with felony aggravated assault, but that charge was reduced Thursday to a misdemeanor.

In Arizona, the felony charge is levied on an adult assaulting a minor under the age of 15, but the teen Arteaga is accused of punching was 15 on the date on the assault, Jan. 30. The boy was deported to Mexico the same day.

A video of the incident allegedly shows Arteaga punching the boy in the stomach after finding him in his cell using a cellphone, which detainees are not allowed to have, according to the Los Angeles Times. After the punch, the boy lies in a fetal position on the floor of the cell.

Documents that came to light on Thursday show that a senior special agent with the Office of Professional Responsibility -- the agency that investigates officer misconduct -- saw the video and acknowledged that Arteaga had punched the teen.

That agent reported the incident to the U.S. attorney's office and on Feb. 12 a prosecutor also watched the video of the alleged assault. In March, federal prosecutors decided not to take on the case.

According to Thursday's report on the incident, the prosecutor said, "[it] did not appear egregious enough to warrant prosecution."

The case was then handed to internal affairs at Customs and Border Protection, which gave its findings to the Santa Cruz County attorney's office, leading to the charges against Arteaga.

The Border Patrol agent turned himself in and was released after being book. Arteaga has been placed on administrative leave. Arteaga's attorney said that his client's actions were "necessary and proper to perform his duties," according to court documents.