A Texas man is accusing a Customs and Border Protection agent of fatally shooting his pet dog during a sweep for undocumented migrants at his home — but the feds say the pooch was at fault because it was too hostile.

Although the shooting went down in early September … the story of what happened stayed under the radar until this past weekend and TMZ just got an update from the Department of Homeland Security, which oversees CBP.

A spokesperson for DHS tells us … CBP agents entered the El Paso residence with consent from the homeowner. While they were inside, the owner’s dog, named Chop, “aggressively charged [at] the agent” and “fearing for his safety, the officer discharged their firearm.” DHS says its is currently reviewing the matter.

Through his attorney, Chop’s owner, Seth Daw, says he’s absolutely heartbroken over the sudden death of the family’s 7-year-old Rottweiler because he believes the killing was “senseless, cruel, and entirely avoidable.”

The attorney, Marisa Ong, says “Border Patrol agents entered a private home under the pretense of an investigation, took advantage of the homeowner’s cooperation, and shot a restrained, non-threatening dog.”

Ong also claims Seth warned the Border Patrol agent about Chop, and the agent put the canine in the bathroom for its own safety while the feds searched the rest of the home.

But, Ong says, Border Patrol dismissed the warning and opened the bathroom door, letting Chop out to get shot by the agent. CBP agents reportedly visited Seth’s house after they received a tip about the home’s previous owner.