Kevin M. Maxwell, chief executive officer of Prince George's County schools. (Marvin Joseph/The Washington Post)

Maryland education officials plan to bring in an independent investigator to examine claims that one of the state’s largest school systems fraudulently inflated its graduation rates by tampering with student grades and credit counts.

The Maryland State Board of Education voted unanimously Tuesday to launch an external review of allegations of “widespread systemic corruption” in the Prince George’s County school system.

“These are serious allegations, and we want to investigate, and we don’t want this to linger,” said Andrew Smarick, president of the state board, who emphasized that the panel has not drawn any conclusions.

The state board’s action coincided with a letter Tuesday from 10 of the school system’s leaders to the state superintendent, urging that an investigation be undertaken.

Kevin Maxwell, chief executive of Prince George’s schools, and a nine-member county board majority wrote that the issue had produced “intense public speculation” and requested an examination “in the interest of transparency and accountability, to bring closure to this matter.” Maxwell denied a systemic effort to manipulate graduation rates.

On Sunday, Gov. Larry Hogan (R) asked the state board to conduct a “complete, thorough and exhaustive” investigation into the allegations of fraudulent practices. A Hogan spokeswoman on Tuesday called the state board’s vote “a positive step forward toward fulfilling the governor’s request.”

The claims were first made by a four-member minority bloc of the Prince George’s school board in a May letter to Hogan that came to light this month.

The four said whistleblowers have come forward with evidence that grades were changed and that students were credited with courses they had not taken. They also alleged that hundreds of students had graduated without meeting state requirements.

Maxwell denounced the allegations, calling them baseless and politically motivated.

In an interview Tuesday, Maxwell restated his view that there was no truth to allegations of systemic corruption and said the county looked forward to the independent examination that the state would arrange.

“We’d like to put an end to this,” he said. “This matter was investigated once before, and there was nothing to substantiate almost exactly the same allegations.”

State education officials conducted an investigation of graduation rates in Prince George’s several months ago after an anonymous complaint to federal officials. The state review included several hours of interviews with Maxwell and four others he appears to have had a hand in selecting. It did not find evidence of improper actions, and federal officials closed the case.

But as the new allegations have drawn attention, many have wondered whether the earlier investigation was sufficiently impartial or broad, and whether it included teachers or others based in schools.

State lawmakers from Prince George’s last week asked for a more in-depth review by the Maryland State Department of Education, including an audit and records analysis.

Hogan’s request turned up the pressure for a second investigation.

“I am deeply troubled by these allegations . . . which have now been brought to my attention by multiple sources,” Hogan said in a letter Sunday.

Maxwell has cited a rise in the county’s graduation rate as one of his signature accomplishments. The rate has climbed more than seven percentage points since 2013, to 81.4 percent in 2016 — the largest increase for that period of any school system in the state.

Despite the improvement, the county’s rate lagged behind the statewide rate of 87.6 percent and the rates in all but one Maryland school system.

The issue has added to tensions between Maxwell, appointed in 2013 by County Executive Rushern L. Baker III (D) to turn around a low-performing school system, and critics on the county board. Recently, Baker announced that he is seeking the Democratic Party nomination in the 2018 race for governor.

State education officials said Tuesday that the process of hiring the independent investigator would begin soon and that the outside organization they hire will report to the state superintendent of schools and the state board.