Chapman further argued that the terms used in Bowen's texts were "commonplace throughout the Border Patrol's Tucson Sector, that it was part of the agency's culture, and therefore said nothing about Mr. Bowen's mindset".
The boy, whose name has not been released, was taken into custody after crossing the southern border in Texas' Rio Grande Valley on May 13, according to the agency.
The agency said the teen, identified as Carlos Gregorio Hernandez Vasquez, was due to be moved to a facility for youth run by the Health and Human Services Office of Refugee Resettlement.
The FBI is investigating the circumstances of the boy's death, the official said.
"The men and women of US Customs and Border Protection are saddened by the tragic loss of this young man and our condolences are with his family", said Acting Commissioner John Sanders in a statement.
The Guatemalan foreign ministry requested that U.S. authorities urgently explain the cause of death, which local and federal law enforcement are investigating.
In San Bernardino, long a transit hub for east-west travel and freight, immigrants were dropped off at the bus terminal by Customs and Border Patrol Agents to wait for family, friends or volunteers to pick them up.
On the morning of December 3, 2017, a CBP camera operator recorded Bowen hitting 23-year-old Antolin Lopez Aguilar. a Guatemalan illegal immigrant, twice with his auto as the man ran away from agents on the American side of the border toward the point of entry.
The man Bowen hit was sentenced to 30 days in a federal prison for illegally crossing the US border. From October 2018 through this April, almost 293,000 unaccompanied children or people traveling in families were apprehended at the southern USA border - almost four times the number during the same period the prior year. This marks the fifth time since December where a migrant child died while in U.S. custody.
Another text about migrants who threw rocks at agents, causing one officer to fall and suffer a gash to his arm, called them "subhuman" and "unworthy of being kindling for a fire".
That has in turn strained USA border facilities, which are the first stop for migrants after they are detained. One Guatemalan man told Reuters that he and his 9-year-old son had spent almost two weeks in Border Patrol custody in Texas, sometimes sleeping on the ground. Border authorities transfer approximately 70 people to hospitals and urgent care centers daily.
"It is appalling that children continue to die on US Border Patrol's watch", the American Civil Liberties Union said in a statement.
Trump administration officials have said they have passed a "breaking point" in the immigration detention system, with the numbers of parents and children crossing the border dramatically exceeding the capacity at facilities.
"There certainly need to be conditions that do not include lying on a mat with a Mylar blanket on a floor that is cold, and cage-like fencing that extends to the ceiling", she said on a conference call with reporters on Monday.
"We absolutely need pediatric health experts at the border".