Almost half of those arrested in a mass Immigration and Customs Enforcement [ICE] roundup of illegal immigrants in Mississippi on Wednesday have already been released, officials announced Thursday.

Immigration and Customs Enforcement (ICE) arrested 680 “removable aliens” at several food processing plants in Mississippi, in what had been deemed the largest single-state immigration enforcement operation in U.S. history.

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However, by Thursday almost half of those detained had been processed and released back where they were initially picked up by authorities.

“Preliminarily, it appears that approximately 30 detained aliens were released yesterday on humanitarian grounds at the individual sites where they were initially encountered, and another 270 detained aliens were released after being processed by [ICEs Homeland Security Investigations] at the National Guard base in Pearl and returned to the place where they were originally encountered,” the U.S. Attorney’s Office for the Southern District of Mississippi said in a statement.

The office then outlines how, according to HSI’s procedures, those detained were asked if they had any children who need to be picked up and made it possible for those detained to contact family members and address related issues.

“As part of HSI procedures pursuant to this operation, if HSI encountered two alien parents with minor children at home, HSI released one of the parents on humanitarian grounds and returned that individual to the place from which they were arrested,” the statement said.

“HSI similarly released any single alien parent with minor children a home on humanitarian grounds and physically returned that person to the place where he or she was originally detained. Based on these procedures, it is believed that all children were with at least one of their parents as of last night,” it said.

ICE had said in a statement late Wednesday that those unlawfully present were being interviewed to note any “potential mitigating humanitarian situations” and the agency was determining on a case-by-case basis who would be detained and who would be released.

In an email to The Associated Press, ICE spokesman Bryan Cox said that those released "were placed into proceedings before the federal immigration courts and will have their day in court at a later date."

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Officials had said Wednesday that they would release detainees who met certain conditions, such as pregnant women or those who hadn't faced immigration proceedings previously.

Such mass-scale raids were more common under President George W. Bush, but such operations were avoided under President Barack Obama. President Trump has resumed those kind of operations, although they are still performed rarely.

Last year, the administration hit a landscaping company near Toledo, Ohio, and a meatpacking plant in eastern Tennessee. The former owner of the Tennessee plant was sentenced to 18 months in prison last month.

As many as 600 ICE agents hit plans in Bay Springs, Carthage, Canton, Morton, Pelahatchie and Sebastpol, and surrounded perimeters to stop suspects from escaping.

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But the raid was also subject of intense criticism from immigration rights groups, 2020 Democratic presidential hopefuls and a number of media reports focusing on the distress faced by the children of those detained.

President Trump has made enforcing immigration law and deporting those in the country illegally a key aim of his presidency -- but he has expressed frustration with policies and laws in place that mean are caught and then released back into the homeland.

Fox News' Vandana Rambaran and The Associated Press contributed to this report.