US airlines on Wednesday spoke out against the Trump administration's "zero tolerance" immigration policy that resulted in the separation of migrant children from their families.

American Airlines, United Airlines and Frontier Airlines all issued statements in the hours before President Donald Trump backed down and signed an executive order to end the controversial policy. Delta Air Lines (DAL) commended Trump for reversing course.

American, United and Frontier earlier on Wednesday asked the federal government not to use their planes to transport migrant children after they're taken from their parents.

American Airlines said it has "no desire to be associated with separating families, or worse, to profit from it. We have every expectation the government will comply with our request and we thank them for doing so."

Like many other US airlines, American Airlines has contracts with the US government to allow federal employees to use its planes for certain travel. For example, the company said it has "carried refugees for non-profits and the government."

None of the airlines were able to definitively say whether their planes had been used to transport migrant children away from their families.

One airline official told CNN that they have limited insight on the exact circumstances of the people for whom the government buys tickets. The airlines don't know whether the individual is being deported, is a child separated from its family and headed to a detention center, or if it's an unaccompanied minor being reunited with family.

As one airline official told CNN, "we don't have full visibility beyond who they are transporting."

The Department of Homeland Security said it was disappointed in the airlines' statements.

American (AAL) said it would be "extremely disappointed to learn" that migrant children who had been separated from their families had flown on its planes.

"[T]he government does not disclose information about the nature of the flights it takes or the passengers who are traveling," the company said.

Frontier said in a brief statement posted to Twitter that it "will not knowingly allow our flights to be used to transport migrant children away from their families."

"At this time, we are not aware if Frontier has been used for this purpose," Frontier's statement reads.

United Airlines (UAL) said it has "serious concerns" about the immigration policy, and said it has "contacted federal officials to inform them that they should not transport immigrant children on United aircraft who have been separated from their parents."

However, the company added, "[b]ased on some research we have done internally and public reports, we have not seen evidence these children have been flown on United aircraft."

The companies did not immediately respond to a request for additional comment.

The news coverage about the policy has included stories of cries from children asking for their parents, harsh living conditions and understaffed facilities.

Related: These states are pulling National Guard troops from the border until family separations end

A Facebook post began circulating last week that claims to be the story of a flight attendant on an unnamed airline who watched 16 migrant children board a flight from Arizona to Miami just after midnight.

Taylor Garland with the Association of Flight Attendants, the largest union for flight attendants, told CNN there are an increasing number of flight attendants sharing experiences of what they believe to be immigrant children.

"The children often do not speak English and appear to have no luggage and look scared. And yes, some are crying," Garland said.

Department of Homeland Security spokesman Tyler Houlton claimed American, United and Frontier want to distance themselves from a number of DHS's missions.

"It's unfortunate that American, United, and Frontier Airlines no longer want to partner with the brave men and women of DHS to protect the traveling public, combat human trafficking, and to swiftly reunite unaccompanied illegal immigrant children with their families," Houlton's statement reads.

Houlton blamed a "false media narrative" for prompting the companies to speak out against the Trump administration's policy.

Shortly after American, United, Frontier and DHS issued their statements, Trump reversed course and signed an executive order to end the family separation policy Wednesday afternoon.

Delta Air Lines issued a statement shortly after the executive order was signed. It criticized the "zero tolerance" policy, but praised Trump for "resolving the issue."

"Recent reports of families being separated are disheartening and do not align with Delta's core values," Delta's statement reads.