* Ex-head of pension agency declines to testify

* Senators seek Justice Dept probe

* U.S. pension agency deficit triples to $33.5 billion

* Unfunded benefits in auto sector are $42 billion (Adds senators seeking Justice Dept probe in paragraph 6)

By Kim Dixon

WASHINGTON, May 20 (Reuters) - The former head of the U.S. agency that insures corporate pensions refused to testify Wednesday at a Senate hearing examining allegations that he had improper contacts with Wall Street firms.

Charles Millard, the former director of the Pension Benefit Guaranty Corp, invoked his Constitutional right to avoid self-incrimination after being subpoenaed to testify at a Senate Aging Committee hearing.

"Against the advice of senior leadership, he participated directly in picking the winners" of contracts that eventually went to BlackRock, JPMorgan and Goldman, PBGC Inspector General Rebecca Anne Batts told the hearing.

The contracts were part of a plan PBGC agreed to in early 2008 to boost its returns by investing in a broader range of instruments, including private equity, real estate and international equities.