The IG's letter in response to a request from Democrats on the committee was sent last week but posted by the Energy and Commerce committee democrats on twitter Tuesday.
The EPA has called the booth a "SCIF", a Sensitive Compartmented Information Facility, and said Pruitt needs the secure line to make calls about classified information and communicate with the president. But he said it's difficult to estimate how often he will need it when he was asked about it while testifying during a House Energy and Commerce Committee hearing last week.
"The use of a secure phone line is strongly preferred for cabinet-level officials, especially when discussing sensitive matters," EPA spokesman Jahan Wilcox said in an email Wednesday, adding that they don't comment on matters involving the inspector general.
But Inspector General Arthur A. Elkins also wrote that he won't be able to start the inquiry right away because the office doesn't have enough resources to keep up with all the investigations and other legally required duties like a semiannual report to Congress.
"The fact is that the OIG has been funded at less than the levels we deem adequate to do all the work that should be done, and therefore we have to make difficult decisions about whether to accept any given potential undertaking," Elkins wrote in the letter.
Each federal agency has an inspector general that serves as a watchdog to investigate complaints about potential fraud, waste, or abuse of agency funds. The EPA's inspector general began looking into complaints about the administrator's travel in August, which attracted scrutiny after reports that at least three cabinet secretaries took private flights that ultimately led then-Health and Human Services Secretary Tom Price to resign.
It will also look into whether Pruitt followed the proper procedures when traveling back to his home state of Oklahoma after complaints that he went there too frequently in the first few months of his time as administrator.