Matthew Klein, the acting commissioner of CBP's Office of Professional Responsibility, said the probe was not considered a criminal investigation at this point but rather an internal investigation to determine whether the employees were involved in "serious administrative misconduct."
Klein declined to identify any individuals involved or whether all 70 people would be punished.
"To be clear, the expectations of professional conduct don't end at the end of one's shift," Klein told reporters. "CBP has set standards of conduct that prohibit the types of posts we saw in this case. CBP has also made it clear to employees that messages posted on a private social media page that are discriminatory, harassing or offensive are not protected and could violate CBP's standard of conduct, and importantly, may have a nexus to the workplace."
Officials launched the investigation on July 1, after a report by ProPublica revealed the posts in the Facebook group known as "I'm 10-15," which included some 9,500 members of current and former Border Patrol personnel. Some posters responded to a teen boy's death in U.S. custody with comments of "oh well' and "if he dies, he dies," while another post questioned whether the image of a father who died trying to save his daughter in the Rio Grande River was authentic.
An internal CBP memo by Klein showed that he had been aware as early as February 2018 of at least one private Facebook group that included "inappropriate and offensive posts" by its personnel. It was not clear whether the Facebook group mentioned in the 2018 memo was "I'm 10-15." In the all-hands memo at the time, Klein noted that the leadership was aware of "inappropriate and offensive posts" and said it must stop.
"The bottom line is the Agency may bring discipline against an employee who posts offensive messages on a social media page where there is nexus to the Agency workplace," Klein wrote in 2018, a statement similar to what he said Monday.
Democrats have accused Trump of enabling a toxic culture to fester at Border Patrol stations, where agents are overwhelmed and under resourced.
Following the ProPublica report, Trump told reporters he hadn't seen the posts, but he also didn't condemn them.
"I will say this. I think that the Border Patrol has been treated very, very badly by certain members of Congress, very, very badly," the president said on July 5. "Certain Members of Congress say very bad things and lie and exaggerate, and Border Patrol people are tough people. They're not happy about it."
ABC News' Karen Travers contributed to this report.