The Bureau of Alcohol, Tobacco, Firearms and Explosives traced one of the three guns used in the Feb. 15 slaying of a U.S. Immigration and Customs Enforcement agent to Otilio Osorio. Prosecutors say he bought the gun in the Metroplex in October.
The Osorio brothers and Morrison will remain in federal custody as they await trial.
"My client is a very young man who's never been in trouble before," said Camille Knight, Otilio Osorio's defense attorney. "He is scared. This is his first brush with the judicial system, and, obviously, this case is about bigger than solely the issues presented at today's hearing."
In court Friday, prosecutors played hidden camera video from a confidential informant who had met the Osorio brothers to transport 40 firearms for them to Mexico.
Federal agents stopped the informant in Laredo and seized the guns, which all had obliterated serial numbers.
Investigators later found more firearms and ammunition inside the Osorios' home, prosecutors say.
A probable cause hearing for Ranferi Osorio was postponed to Wednesday.
Both Osorio brothers are charged with possessing firearms with an obliterated serial number. If convicted, they each face up to five years in prison and a $250,000 fine.
In a separate criminal complaint, Morrison is charged with knowingly making false statements in connection with the acquisition of firearms and dealing in firearms without a license. If convicted of both counts, he faces up to 15 years in prison and a $500,000 fine.