Calpers, the biggest U.S. public pension fund, is reviewing its business relationship with Apollo Global Management, examining fees, performance and the relationship as a whole, the Wall Street Journal reported on Thursday, citing documents outlining the review.

The review was started in May and is still active, the paper reported. Calpers, the California Public Employees' Retirement System, has hired outside adviser Houlihan Lokey Howard & Zukin to analyze the relationship and to offer alternative courses of action, it said.

The Wall Street Journal received access to the documents through a public-records request.

Calpers' relationship with Apollo has become subject to outside scrutiny since the pension fund said last week that it is probing fees paid by outside money managers to win its business.

The pension fund listed several of Apollo's funds as having paid fees to ARVCO Financial Ventures, a firm headed by former Calpers board member Al Villalobos.

Calpers and Apollo could not be immediately reached for comment.

A Calpers spokeswoman told the Journal that the review was part of a larger strategy.

This is not an effort to end our partnership and we have full confidence in Apollo, she said.

(Reporting by Michael Erman; Editing by Lincoln Feast)

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