WASHINGTON — A federal watchdog is investigating a program that spent $38 billion under ex-Homeland Security Secretary Kristi Noem to convert vacant buildings into immigrant detention facilities, The Post has learned.

The Department of Homeland Security Office of Inspector General (DHS OIG) has announced the probe “[t]o determine to what extent U.S. Immigration and Customs Enforcement [ICE] purchased and converted facilities for detention space that meets its operational need in a cost-effective manner.”

ICE’s acquisition drive — which was also championed by Noem’s top adviser Corey Lewandowski — has spent just $1 billion so far in taxpayer funds to purchase 11 empty warehouses, The Wall Street Journal first reported.

In total, up to $45 billion was appropriated “for single adult alien detention capacity and family residential center capacity” in the One Big Beautiful Bill signed into law by the President this past July.

ICE bought the warehouses from sellers like Goldman Sachs and the Carlyle Group, as opposed to leasing facilities from private prison companies, per the real-estate analytics company CoStar.

In a March report, CoStar alleged that DHS paid premiums of between 11% and 13% over the market price for similar properties.

Some companies that received federal contracts related to the building purchases had never previously worked on immigration matters, according to The Journal.

Noem’s replacement as DHS secretary, Markwayne Mullin, paused spending on the warehouse-to-detention facilities program shortly after assuming the cabinet post in late March.

The IG probe follows a similar inquiry into Noem and Lewandowski’s approval of hundreds of millions of dollars in federal contracts in fiscal year 2025 — including several no-bid pacts.

“DHS OIG can confirm this audit was announced recently,” a rep said.

“However, as a matter of course, we do not provide any additional information publicly on the scale or scope of our ongoing work.”