The Trump administration put detained immigrants into solitary confinement more often than the Obama administration during a similar time period, according to Immigration and Customs Enforcement records obtained by the Project On Government Oversight, a Washington watchdog group.

ICE records show a 15.2 percent increase in the number of placements in solitary confinement during the first 15 months of the Trump administration, versus the same span at the end of the Obama administration.

"Under President Trump, ICE is confining far more detainees in solitary — of whom a strikingly high proportion have mental illness — than at the end of the Obama administration," said Nick Schwellenbach, director of investigations at POGO, and co-author of a report published Wednesday analyzing ICE’s use of isolation.

Click here to read the POGO report

While the exact number of detainees is unclear because there are multiple placement reports for some individuals, the number of placements grew from 3,344 for the last 15 months of the Obama administration to 3,855 in the first 15 months of Trump, according to an analysis of the records obtained by POGO.

The POGO report says that figures for the first third of 2018 indicate the number of solitary placements was continuing to increase.

ICE tracks solitary confinements only when detainees have a "special vulnerability," such as physical disabilities or mental illness, or were put in solitary for more than 14 days.

The POGO report says that the number of placements that involved a detainee with mental illness was 40 percent in 2016 and 39 percent in 2017, and was on track for a similar percentage in 2018.

The POGO report comes several months after an NBC News investigation in partnership with the International Consortium of Investigative Journalists and the Intercept documented thousands of instances in which ICE detainees were put in solitary confinement.

The stories sparked several Democratic and Republican senators to press ICE for answers about how the agency uses solitary confinement.

"It is imperative that ICE swiftly resolve any lacking oversight or improper documentation pertaining to the use of segregation," Sen. Chuck Grassley, R-Iowa, wrote in a joint letter with Sen. Richard Blumenthal, D-Conn., to ICE dated July 24.

The previous stories by NBC News and its partners documented a range of questionable practices, including how dozens of detainees were put in solitary confinement simply because they suffered from physical disabilities and needed a prosthetic leg, a cane or a wheelchair.

Detainees with mental illnesses were put in solitary, often for weeks or months. LGBT detainees were also routinely put in isolation solely because of their sexual orientation. Only half of the time were detainees put in solitary because of disciplinary violations, according to ICE records.

Blumenthal has called for Senate hearings. Two other Democratic senators, Elizabeth Warren of Massachusetts and Cory Booker of New Jersey — both presidential contenders — have also raised concerns. Warren has asked ICE a series of questions while Booker, a member of the Senate Judiciary Committee, which oversees ICE, has also called for hearings.

The earlier reporting by NBC News and its partners was based on ICE documents and data obtained through Freedom of Information requests. The POGO report released Wednesday adds data up through May 4, 2018, also obtained through FOIA.

Bryan D. Cox, an ICE spokesman, declined to comment on the report because the agency has not had an opportunity to review it, but told NBC News, “Any suggestion that the use of segregation in ICE custody is above the norm for detained populations would be a false claim.”

He said the agency conducted studies in 2012 and 2013 that show ICE put detained immigrants in isolation “at a rate significantly below the national average” for criminals in prisons.

“ICE provides several levels of oversight in order to ensure that detainees in ICE custody reside in safe, secure and humane environments and under appropriate conditions of confinement,” Cox said.