U.S. Secretary of Defense Jim Mattis clashed with U.S. Sen. Kirsten Gillibrand (D-N.Y.) while trying to justify President Trump’s transgender military ban during an Armed Services hearing on Thursday.

In response to a question from Gillibrand, Mattis claimed that Obama Secretary of Defense Ash Carter had not consulted any of the four military services’ top generals nor the Chairman of the Joint Chiefs of Staff before lifting the prohibition on transgender service members in 2016.

Mattis told Gillibrand that the directive issued under Carter was “opaque” and blocked the Pentagon from revealing problems associated with allowing transgender people to serve openly, reports the military publication Stars and Stripes.

As he attempted to defend the administration’s policy, released last month, which prohibits most transgender individuals from serving,. However, if a transgender person has not undergone gender confirmation surgery or has not been diagnosed with gender dysphoria, they may be allowed to serve so long as their presence will not affect military readiness. That revised policy is currently being challenged in the courts, with at least one federal judge keeping in place an order that prevents the Pentagon from implementing the policy until it is determined to be constitutional.

Despite four ongoing lawsuits challenging the ban, which Pentagon officials insist limits what they can say about the policy, the service chiefs of the Army, Air Force, Navy and Marine Corps have previously testified before the Senate Armed Services Committee that they have not received reports of any issues involving transgender servicemembers.

“We cannot report that a problem emanated from a transgender” person,” Mattis said. “We cannot under the Carter policy do that. So the question you’ve asked the service chiefs and the chairman are ones that, right now, the Carter policy prohibits that very information from coming up, because it’s private information. It’s impossible for them to respond to you.”

But Gillibrand blasted both Mattis and Marine Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, claiming that they had based the revised ban on flawed science and comparing the ban on transgender troops to past bans that had prohibited other classes of Americans from serving.

“It appears that this report your department has issued is not based on the department’s data or science, but rather on quote ‘potential risks’ that the authors cannot back up,” Gillibrand said. “In fact, this seems to me to be the same unfounded claims and unfounded concerns that led the opposition to repealing ‘Don’t Ask Don’t Tell,’ integrating women into the military, integrating African Americans into the military, and I think you need to do a lot more work on this topic to inform yourselves.”

Gillibrand noted that a study — using Pentagon data — by the Palm Center, a think tank that advocates for allowing LGBTQ people to serve openly, had found that transgender troops could easily deploy without significant problems. That study found that nearly 400 servicemembers, out of a total of 994 diagnosed with gender dysphoria, had served in Iraq or Afghanistan in 2016 and early 2017. Of those 400 serving, only one had a gender dysphoria-related incident during the deployment.

The Trump administration has also been criticized by six former U.S. surgeons general, who sent a letter to the Defense Department arguing against a ban. The surgeons general accused Mattis and other top Pentagon officials of having “mischaracterized the robust body of peer-reviewed research” on gender dysphoria and transgender medical care.

The American Medical Association has previously made similar claims, finding, both in 2015 and earlier this year, that there is “no medically valid reason” to exclude transgender troops from serving openly — something the surgeons general cited in their letter.

“In light of last month’s announcement concerning military policy for transgender servicemembers,” they wrote, “we underscore that transgender troops are as medically fit as their non-transgender peers and that there is no medically valid reason — including a diagnosis of gender dysphoria — to exclude them from military service or to limit their access to medically necessary care.”

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