More than 600 illegal immigrants with criminal convictions were released by federal authorities in early 2013 in advance of the "sequester" budget cuts, according to a government watchdog report obtained by FoxNews.com.

The report by the Department of Homeland Security Office of Inspector General showed that Immigration and Customs Enforcement released "criminal aliens" from jails all over the country as part of a wave of releases in February 2013. The agency came under fire at the time for the decision, which critics described as politically motivated and dangerous.

The report, obtained in advance by FoxNews.com and released late Tuesday morning, found that ICE indeed was trying to address the budget shortfall when leadership moved to sharply reduce the number of people in the detention centers.

But in releasing more than 2,200 detainees, the agency reported it also had sprung 629 with criminal convictions (the agency later lowered that estimate slightly) -- some of whom had been nabbed on serious offenses. The report indicated many should not have been released.

"Between February 9 and March 1, 2013, Enforcement and Removal Operations field offices released some aliens with criminal convictions whose detention was statutorily required," the report found.

The probe was requested by Sens. John McCain, R-Ariz., and Tom Coburn, R-Okla., who slammed the agency over the findings.

"It is baffling how an agency charged with homeland security and immigration enforcement would knowingly release hundreds of illegals with criminal histories. In this single action, ICE undermined its own credibility, the rule of law, and the safety of Americans and local law enforcement," Coburn said in a statement.

The report said that in advance of the highly publicized budget cuts -- which the Obama administration was publicly opposed to -- ICE released 2,226 immigration detainees.

Most of the hundreds with criminal convictions were categorized as "level 3" -- meaning they were implicated in lower-level offenses.

But the group included dozens of "level 1" and "level 2" inmates. Level 1 is a category that includes those convicted of offenses ranging from murder to rape to robbery to serious drug crimes. Level 2 includes those convicted of fraud, burglary, money laundering and other offenses. The IG report did not say which offenses these individuals were being held on.

The report called the agency's approach here "problematic," saying ICE leadership did not "communicate effectively" with the division in charge of detentions and did not inform higher-ups about the budget shortfalls.

The report described an inconsistent and confusing scramble to make space in the detention centers. Local divisions did not have written guidance on how to "prioritize" the releases, and had "limited access" to legal advice since ICE leadership "did not formally notify ICE attorneys" of the plan. Only one local field office had an attorney on hand to review "each case," according to the report.

Yet the report noted that many of the illegal immigrants were required by law to be held. Adding to the confusion, ICE leaders did not tell local field offices to track the releases. When they started asking questions, officers had trouble providing the information.

In the end, while the agency found officers made "reasonable release decisions" given the time frame, they ended up redetaining dozens of those they had released.

In April 2013, the agency reported that 34 "level 1" detainees had been released, but 24 of them were returned to ICE custody. Thirty of the 80 "level 2" inmates were also brought back.

In a written statement, ICE said the reductions in 2013 were "necessary" due to the "high levels of detention at the time, coupled with the looming prospect of sequestration."

ICE said the agency took "careful steps to ensure public safety by focusing efforts to reduce the detained population on non-criminal, non-mandatory detention cases and cases involving individuals who posed no serious risk to community safety." The assessments, according to ICE, were made by "career law enforcement officials on a case-by-case-basis following a careful examination of each detainee's criminal and immigration history."

The agency said those released remained in removal proceedings and "continue to be monitored."

Then-ICE Director John Morton earlier acknowledged to Congress in March 2013 that those released included individuals with criminal backgrounds. He said at the time the agency was bringing some of them back into custody, but claimed some had been held just on financial offenses.

Morton stressed that the releases were made on a case-by-case basis and not "willy-nilly."

But Republicans have long voiced concern that the agency, in this case and others, has put local communities at risk with its releases. The IG report comes as the agency, along with other federal departments, grapple with the surge of illegal immigrant children crossing the U.S.-Mexico border.

The IG report did note, however, the constraints ICE is facing. The agency is required by law to maintain an average daily population of 34,000 detainees, with funding coming from Congress and other sources. But at the time, the report said, ICE was dealing with an average population of more than 35,000 detainees; facing budget cuts and other fiscal restraints, the agency decided to reduce the number of prisoners.