STELLENBOSCH, South Africa (AP) — Chicago Marathon champion Rita Jeptoo failed a doping test in September, the Kenyan runner's agent said Friday.

Federico Rosa told The Associated Press that Jeptoo tested positive for a banned substance in an out-of-competition test in Kenya weeks before she won a second straight Chicago Marathon title on Oct. 12.

"This is true," Rosa said in a telephone interview.

Rosa said Jeptoo's "A'' sample had tested positive and they were waiting for her backup "B'' sample to be tested. Rosa declined to name the substance.

Rosa said he had learned of the positive test "a few days ago."

"We will legally go after the person or the people that convinced Rita to do this," Rosa said, adding Jeptoo's management had "nothing to do with" any doping. "I am sorry for Rita."

The 33-year-old Jeptoo has won back-to-back titles at the both the Chicago and Boston Marathons and leads the World Marathon Majors series.

The series said earlier Friday that it had postponed Sunday's awards ceremony because of the positive test.

"No athlete can win the World Marathon Majors Series title who has been in breach of IAAF anti-doping rules," the series said.

The IAAF said in a statement that it was "not in position where it can confirm or deny" Jeptoo's positive doping test.

"The case remains in the confidentiality phase," the IAAF said, "although that should be lifted within the next week."

Jeptoo's failed doping test comes as Kenya is under close scrutiny for a recent spike in doping cases.

A report into doping in Kenya was released this month. The investigation found that the East African country has problems with athletes from a number of sports using banned substances.