A bunk bed and desks inside a cell is seen at the Caroline Detention Facility in Bowling Green, Virginia, Aug. 13, 2018. SAUL LOEB/AFP/Getty Images

The recent statistics from Immigration and Customs Enforcement (ICE) showed that the number of detainees at the agency's various detention centers has hit a record high of 44,631 people.

According to The Daily Beast, this increase has prompted the rights groups to question how the controversial agency obtained the money to place 4,000 more people into custody than Congress has funded. When ICE faced fund shortfall earlier this year, the Department of Homeland Security quietly moved nearly $100 million out of other areas of its budget, including the Federal Emergency Management Agency.

The decision resulted in an uproar from Oregon Democrat Jeff Merkley who said it was unsurprising that President Donald Trump's administration was “exceeding historic high water marks of detainees to pursue their ideologically driven policy agenda.” He also demanded ICE account for how it had somehow found the money.

“It is incredibly important that ICE explain how they’re paying for nearly 4,000 more beds. In September, when I discovered that ICE had been reprogramming FEMA dollars to pay for immigrant detention centers, I wasn’t given the information from the administration. I wasn’t given the information as a member of the Senate appropriations committee. I found the information through outside resources,” he said.

“The plain fact is that the administration never wanted anyone to know how they were planning to pay to execute their plan. They used a mechanism that was never intended to see the light of day,” Merkley added.

The agency’s recent submissions to Congress justifying its budgets show an upward trend in its detention operations. In March, the congressional appropriations committees gave ICE $7.1 billion, its highest budget ever, including $4.1 billion for immigrant removal and detention operations. This was about $401 million more than the amount sanctioned the previous year.

Danielle Bennett, ICE spokesperson, acknowledged that Congress funded ICE for 40,520 average daily detainees this year “though ICE does have the flexibility to go above that number.”

“Ensuring there are sufficient beds available to meet the current demand for detention space is crucial to the success of ICE’s mission. Accordingly, the agency is continually reviewing its detention requirements and exploring options that will afford ICE the operational flexibility needed to house the full range of detainees in the agency’s custody,” Bennett added.

According to the ICE budget for the fiscal year 2018, on average it costs $133.99 a day to maintain one adult detention bed while the cost to maintain a family bed, keeping mothers and children together in a family residential center, costs around $319 a day.

However, as of April, there was an increase in the number of children being separated from their parents. This resulted in the creation of "tent cities" to hold thousands of separated children. The Department of Health and Human Services told NBC that the cost of these beds is $775 per person per night.

An ICE spokesman told CNBC in June that it oversees "juvenile facilities," managed by local jurisdictions that allow for "temporary housing of children separate from adults." This costs $139.40 per day per person.