[Updated (2/4/2016, 5:14 p.m.) with a statement from a lawyer representing Mr. Casares.]

The president-elect of the Association for Student Conduct Administration published an open letter on Wednesday asserting that her predecessor, who recently stepped down, had sexually assaulted her.

The president-elect, Jill L. Creighton, assistant director for global community standards at New York University, said in the letter, posted on Twitter, that Jason Casares “took advantage” of her at a convention last year. She also wrote that she had filed a criminal complaint against him and that she had asked the association to impeach him.

In a letter released on Thursday, the association’s Board of Directors said an independent investigator it had hired to examine the allegations had determined Ms. Creighton’s claims “could not be substantiated,” ending the complaint-resolution process.

“ASCA treats all reports of misconduct seriously and therefore conducted a thorough investigation in accordance with state laws and ASCA governing documents,” the board said. Much of the association’s membership deals with issues of sexual violence on college campuses.

In its letter the association also confirmed Mr. Casares’s resignation, but did not say why he had stepped down.

Mr. Casares, associate dean of students and deputy Title IX coordinator at Indiana University at Bloomington, has been put on paid administrative leave by the university, a campus spokesman said on Thursday.

In her letter, Ms. Creighton said the association had refused to cancel sessions scheduled to be led by Mr. Casares at the group’s annual conference, taking place this week in Florida. “This is not something the association can afford to be ambivalent about,” Ms. Creighton wrote. “We cannot claim national leadership in addressing sexual misconduct, only to fail miserably in our first test within our own association.”

The association said in its letter that it “is working to accommodate the needs of both Ms. Creighton and Mr. Casares during this difficult time, taking into account safety and privacy precautions.”

According to the conference schedule, Mr. Casares was scheduled to present in several sessions, including one titled “Using a Trauma-Informed Approach in Sexual-Misconduct Investigations.”

In a statement, a lawyer representing Mr. Casares, Tony Paganelli, said the allegations were false, and pointed to the association’s finding that Ms. Creighton’s claims were unsubstantiated. “Despite the outcome of the investigation,” he wrote, “Ms. Creighton has made inappropriate and false public statements reasserting her claims against Mr. Casares over social media, and in person to attendees at an ASCA conference this week in Florida.”

Read the letter from Ms. Creighton.

Read the letter from the association.

Read the statement from Mr. Casares’s lawyer.