CHICAGO (CBS/AP) — The second ranking Democrat in the U.S. Senate has called on Homeland Security Secretary Kirstjen Nielsen to resign over the Trump administration’s policy of separating immigrant children from their parents at the border.

“This policy shows the extremes this administration will go to to punish families fleeing horrific gang and sexual violence, and seeking refuge in the United States,” U.S. Sen. Dick Durbin, D-Illinois, said at a Senate Judiciary Committee hearing that saw lawmakers from both parties criticize the controversial family separations.

Calling the Trump administration’s policy a “humanitarian disaster,” Durbin said someone in the Trump administration must be held responsible for a policy that saw approximately 2,700 children forcibly separated from their parents or guardians before courts ordered them reunited.

“The family separation policy is more than a bureaucratic lapse in judgement. It is and was a cruel policy inconsistent with the bedrock values of this nation,” the senator said.

Durbin noted more than 700 children remain separated from their parents after the court-ordered deadline. Of those, about 400 children’s parents were deported before they could be reunited, and the feds have been unable to locate the parents of 90 other separated children.

“What will become of these children and their parents, who border agents called, ‘deleted family units’? In the name of these ‘deleted family units,’ 711 ‘lost children,’ and common decency, I am today calling on the architect of this humanitarian disaster, Department of Homeland Security Secretary Nielsen, to step down,” he said.

Durbin’s demand will likely be ignored, and Homeland Security spokesman Tyler Houlton quickly issued a statement saying Nielsen is “doing the job she was nominated, confirmed and sworn to do” and said “the obstructionists in Congress” should pass laws protecting U.S. borders.

However, Durbin noted top Democrats in Congress already have proposed a number of measures to overhaul border security, including legislation that would specifically prohibit family separations, a plan to provide unaccompanied children with attorneys during deportation hearings, and a measure to prioritize detention of migrants considered threats to national security or public safety.

“We can have border security without bullying. We can be safe without treating toddlers as terrorists,” Durbin said. “We do not need internment camps again in American history.”

The Trump administration’s “zero tolerance” policy of prosecuting migrants entering the U.S. without authorization, and its separation of thousands of children from their parents, has drawn election-year criticism from both parties.

Committee Chairman Charles Grassley, R-Iowa, said Trump’s crackdown on people illegally crossing the border from Mexico was well-intentioned but has had unintended consequences.

He said the administration has “mishandled” family separations. He also cited reports that immigrants have experienced sexual and other abuse at some government detention facilities and said those held must be treated humanely.

Late Monday, Grassley and the Judiciary panel’s top Democrat, Dianne Feinstein of California, sent a letter asking the inspectors general of the Homeland Security and Health and Human Services departments to investigate news organization’s reports of abuse of immigrants at detention centers.

“These allegations of abuse are extremely disturbing and must be addressed,” Grassley and Feinstein wrote. “This is not a partisan issue as reporting suggests many have been occurring for years. Immigrant families and children kept in federal custody deserve to be treated with basic human dignity and respect and should never be subjected to these forms of abuse.”

Trump began a policy of “zero tolerance” this spring, prosecuting all migrants caught entering the U.S. without authorization. To help discourage border crossing, his administration also began separating children from their detained parents, rather than following the policy used by previous administrations, which generally released the entire family pending court action.

Under withering public rejection and criticism from congressional Democrats and Republicans alike, Trump stopped taking children from their parents. But of the more than 2,500 children held, hundreds were not reunited by last week. That includes more than 400 whose parents were deported.

U.S. District Judge Dana Sabraw in San Diego set a deadline of last Thursday to reunite the families. While he commended administration officials for reuniting many parents in its custody with their children, it faulted them for leaving hundreds of families still apart and warning that a better system must be in place.

The senators’ letter, based on articles by The Associated Press and other news organizations, says the allegations suggest “a long-term pattern” of mistreatment. Those reports describe claims of abuse from this year dating back to before Trump took office.

The AP reported last month that children held at an immigration detention facility in Roanoke, Virginia, said they were beaten while handcuffed, locked in solitary confinement and left nude and cold in concrete cells.

A civil rights lawsuit has been filed alleging mistreatment at the Shenandoah Valley Juvenile Center from 2015 to 2018. The alleged victims, Hispanic youths held for months or years, have submitted sworn statements in the case. Lawyers for the facility have denied the alleged abuse.

The senators’ letter also cited a New York Times report this month about two female migrants who described sexual abuse at detention facilities in Texas and Pennsylvania. Immigration and Customs Enforcement has reported 1,310 cases of sexual abuse against detainees from 2013 to 2017, the report said.

A June report by the website Dallas News described alleged sexual abuse at a detention center near Austin, Texas, in 2017. The Arizona Republic reported alleged inappropriate contact involving a teenage boy in 2015 and a girl who accused a staffer of making suggestive comments in 2017 at facilities in Glendale and Tucson, Arizona. The lawmakers’ letter cited those reports as well.

Homeland Security spokeswoman Katie Waldman said agency officials perform their duties “professionally and humanely” and that the agency “is abiding by the intent and letter of law and maintains the highest standards care for individuals in our custody.”

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)