Gov. Larry Hogan (R) has requested a probe into Prince George’s public schools. (Linda Davidson/The Washington Post)

Maryland Gov. Larry Hogan (R) on Sunday asked the State Board of Education for a “complete, thorough and exhaustive” investigation into alleged tampering with grades and graduation rates at Prince George’s County Public Schools.

The allegations were levied this month by four members of the Prince George’s County school board, who said they have evidence to prove their claims but have not made that evidence public.

Kevin Maxwell, chief executive of the county’s schools, denied the allegations and called them a personal attack on all school employees. He said the charges are a politically motivated attempt to undermine recent gains.

In a Sunday letter to the state board, Hogan, who is just back from a trade mission to Europe, said he was deeply troubled by the allegations and asked the board to begin an inquiry immediately. State lawmakers from Prince George’s wrote the state board Thursday with a similar request.

Hogan said whistleblowers in the case should receive “the full protections under the law.”

Prince George’s schools chief Kevin Maxwell speaks while County Executive Rushern L. Baker III (D), left, looks on. (Mark Gail for The Washington Post)

The allegations have increased tensions in Prince George’s between the schools chief, who was recruited in 2013 by County Executive Rushern L. Baker III (D) to overhaul the school system, and Maxwell’s critics on the county board.

Baker announced last week that he is seeking his party’s nomination to run against Hogan in the 2018 gubernatorial race. One of his rivals for the nomination, former NAACP chief Ben Jealous, attended a rally outside a Prince George’s school board meeting Thursday night where participants called for Maxwell to be held accountable.

Maxwell has touted an uptick in Prince George’s graduation ratesas an indication that his policy changes are taking root. An investigation by the state earlier this year, triggered by an anonymous complaint, found no evidence of tampering. But critics say the investigation was not sufficiently broad or impartial.

The Prince George’s school board members have alleged that grades were changed and students were credited for courses they did not take, leading to fraudulent graduation rates.