Environmental Protection Agency chief of staff Ryan Jackson signed off on most of the high-profile pay raises that have landed EPA chief Scott Pruitt in hot water in recent weeks, the EPA inspector general said Monday.

Although Pruitt did authorize tens of thousands of dollars in pay raises for select employees, the biggest pay hikes came at the behest of Jackson, according to the inspector general.

The inspector general's preliminary report was sent to Pruitt as an update on its ongoing investigation into whether the EPA broke the law in giving a group of lobbyists and others jobs at the agency with huge pay raises in the last year.

The highest five-figure pay raises for two aides were authorized by Jackson. He had said last week that he authorized the pay raises, not Pruitt.

[Related: Scott Pruitt says he killed pay raises for staffers, didn't know about them]

The pay raises are part of a growing list of concerns by lawmakers and others over Pruitt's running of the agency and whether the White House should force him to resign. The White House is reviewing Pruitt's actions as well.

In half of the six cases, Jackson had signed off on tens of thousands of dollars in raises using Pruitt's signature, the inspector general said. Two of the employees had increases of between 67 and 72 percent.

"The authorizations for three of these original actions were personally signed by the administrator; the other three authorizations were signed by the chief of staff 'for Scott Pruitt,'" read the IG report. In many of the cases, Jackson had both requested and authorized the changes.

Most of Pruitt's pay raise approvals were for lesser amounts than Jackson's, but still large. Six months after joining the agency, Pruitt signed off on a title change for one employee with a 26.7 percent pay increase.

The inspector general report also noted that the EPA has not given the office any documentation indicating changes to the employees' pay raises. "The agency was unable to provide us with complete information or confirmation of any modifications," it stated.

Pruitt had said in an interview with Fox News that he had killed the massive pay raises for the two aides when he learned about them.

Both Democrats and Republicans are probing the pay raises as well as Pruitt's expensive travel and security expenses.

A separate watchdog report issued Monday by the Government Accountability Office showed Pruitt violated the law by building a $43,000 soundproof phone booth in his office.

The inspector general report released Monday is a preliminary showing of the facts, without any conclusions or recommendations made. The inspector general will continue its audit of EPA's hiring practices, it said.