Rep. Tom Cole, R-Okla., the committee chairman, released a statement that did not go into details. The Hill cited a source that said the FBI and Federal Election Commission has been told of the problems.
The irregularities involved a former staff member doing work as a contractor.
"We are aggressively and thoroughly investigating the matter and, while we determine the details, we have terminated our relationship with a former employee who was engaged as an outside vendor," Cole said.
Republican fund raising this year has fallen far short of the amounts raised by Democrats, leaving the party short of money for its congressional candidates.