Attorneys on Friday struck back against a claim long made by the U.S. government that a landmark 1997 court settlement about the care of migrant children in custody has caused kids and families to flow north from Central America.

Their filing and accompanying declarations by immigration experts is the latest salvo over the Flores Agreement, which set standards for how unaccompanied migrant children and families with kids apprehended together must be sheltered by the government.

In a previous filing in the case from August 31, the government claimed that immigration has changed in the 22 years since the Flores Agreement was first established. The government also claimed that the agreement itself led to an increase in children and families coming to the U.S.

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It further argued that those changes, coupled with a final regulation posted in August that would allow for the indefinite detainment of children, should render the Flores Agreement moot.

Lawyers wrote in Friday's opposition brief that the government is attempting through its August 31 filing to "light a match" to the Flores Agreement, but said the filing relied on a "dizzying array" of claims about migrant children and families that federal courts had already rejected.

One of the attorneys who contributed to the brief, Neha Desai, said in an email to CBS News that "the government has a legal obligation to ensure the basic rights of children in federal immigration custody."

"(The government) has made multiple failed attempts to have the courts permit it to evade this obligation in the past few years. Now, the government has attempted again, this time through the issuance of regulations and through recycled, unsupported arguments including that circumstances have changed such that Flores should terminate," wrote Desai, who is the Director of Immigration at the National Center for Youth Law.

"These attempts are a basic affront to longstanding principles of child welfare, not to mention human decency," she added.

In the brief, filed in the U.S. District Court for the Central District of California, attorneys cited experts who said in attached declarations that complex, local socioeconomic factors cause families with kids — or children traveling without families — to uproot their lives. Those factors include gang violence, extreme poverty and local government corruption.

"Families do not flee their homelands, subjecting themselves and their children to the disruption of displacement and the risks of the journey, for a single motivating factor — least of all an obscure law or policy in a receiving country, such as the Flores Settlement in the United States — that may or may not afford some degree of protection from prolonged detention," wrote Amy Thompson, a postdoctoral scholar at the Colegio de Sonora in Mexico, where she studies migration.

Her observation was echoed in an accompanying declaration by University of Pennsylvania professor Fernando Chang-Muy, who travelled throughout Honduras in May 2019 giving presentations to potential asylum seekers about the legal hurdles they might face in the United States.

"Few to none of them are even aware of the existence of the Flores Settlement Agreement," Chang-Muy wrote.

President Donald Trump and other administration officials have long claimed the Flores Agreement is viewed as a loophole, allowing migrant adults to bring children with them to the U.S. because families cannot be detained at-length. The agreement calls for unaccompanied children to swiftly be placed with "sponsors" – typically relatives. A 2015 opinion by the judge overseeing the Agreement led to a so-called "20-day rule" limiting detainment of families.

The final regulation announced by the government on August 21 would do away with those limits. It is scheduled to go into effect on October 22. Government officials have said they believe the move will deter would-be migrants.

"With the Flores fix... We're looking forward to seeing less and less family units coming across the border," said U.S. Citizenship and Immigration Services Acting Director Ken Cuccinelli in a video posted to Twitter the day the regulation was announced.

It’s #Flores fix day, & I visited ⁦@ICEgov⁩’s largest family residential facility in Dilley, Tx. They are doing a great job here! So impressed. pic.twitter.com/IBOHE2XNOg — USCIS Acting Director Ken Cuccinelli (@USCISCuccinelli) August 21, 2019

However, the government's own data contradicts its assessment of the "20-day rule," according to an analysis submitted Friday by University of California, San Diego political science professor Tom Wong.

"The government's claims about correlation obfuscate the fact that the 2015 Flores ruling came in the midst of an increasing trend of migrant family apprehensions," Wong wrote.

In the last year, immigration enforcement has contended with a surge of families and unaccompanied migrant children fleeing destitution and violence in Central America. In the government's August 31 filing in the Flores case, Assistant Attorney General Joseph Hunt said changes in migration patterns mean it "is no longer possible, equitable, or in the public interest" to continue the Flores Agreement.

That argument was refuted Friday by Susan Martin, Professor Emerita at Georgetown University, whose decades in immigration policy include publishing one of the first studies of unaccompanied refugee children, leading two Congressional commissions and writing the United Nations High Commissioner for Refugees' first guidelines on protection of refugee women.

"I believe that US policymakers were aware of the irregular migration of accompanied children — not just unaccompanied children — when the original Flores Settlement Agreement was negotiated in 1997," Martin wrote. She noted that at the time, "families were the preference" and future waves of migrants were anticipated:

"The original Flores Settlement was not adopted in a vacuum. The federal government was well aware that children were arriving in the United States alone as well as with their family."