A federal border control agency found that the US border agents riding on horses who assaulted Haitian migrants last year used "unnecessary force".
The incident saw riders threaten migrants and force them into the Rio Grande River in Texas. It sparked comparisons with America's slavery era.
Customs and Border Protection (CBP), after an inquiry, concluded that agents likely violated US immigration laws.
At least four agents are likely to face unspecified disciplinary actions.
The long probe was criticised for its slow pace. Investigators blamed a lack access to official communications.
CBP released its Friday findings and did not recommend that any officers be fired, but it acknowledged "failures in making good decisions at multiple levels"
CBP Commissioner Chris Magnus stated that the incident was caused by "failures to control Horse Patrol Units, a lack of policies and training, as well as the chaotic nature of the situation at Del Rio at that time."
He continued, "Several agents engaged with unprofessional or dangerous behavior, including one instance when an agent used demeaning and offensive language."
Magnus said that his agency had already begun to review its training, operations, and practices in relation to the incident. He also stated that he has limited the use of horses patrols.
Agents on horse patrol were deployed to stop a surge in migrants who had crossed an international bridge at Del Rio.
Paul Ratje, an AFP photographer, captured images of Haitians crossing the border to feed their families. He found them blocked by horses.
These images were shared widely online and appeared to show mounted men patrolling with whips, whipping migrants.
Officials refuted this claim, but added that there was no evidence that migrants were denied entry to the US or forced to return to Mexico.
Investigators examined the actions of a few agents in a 30-minute period. They also spoke with 11 attorneys representing asylum-seekers who were involved in the event. However, they did not directly speak with migrants.
Prosecutors in Texas declined to bring criminal charges against the agents involved in this encounter earlier in the year.
