WASHINGTON — Prosecutors are gathering evidence for a grand-jury probe into whether congressional staff helped tip Wall Street traders to a change in health-care policy, an indication the long-running investigation has entered a more serious phase.

Public documents show federal law-enforcement officials and the Securities and Exchange Commission are seeking records and other evidence from the House Ways and Means Committee and a top congressional health-care aide, Brian Sutter, staff director of the Ways and Means Committee’s health-care subpanel.

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The SEC sent subpoenas to the House committee and Mr. Sutter seeking documents and testimony in the matter, according to documents made public by Rep. David Camp (R., Mich.), who is the committee chairman, and Mr. Sutter.

Separately, the Justice Department issued a subpoena to Mr. Sutter to compel him to testify before a federal grand jury at the U.S. District Court for the Southern District of New York, according to Mr. Sutter’s public disclosure, which was included in the congressional record per House rules. Committee officials wouldn’t say whether Mr. Sutter has testified. A spokesman for the Ways and Means Committee, speaking for Mr. Sutter, declined to comment.

The subpoenas are related to criminal and civil investigations examining whether anyone in the government illegally passed along nonpublic information about the health policy that ended up in the hands of traders, according to people briefed on the matter. The probe was sparked by a 2013 Wall Street Journal report that detailed how health-insurance stocks jumped moments before the government announced news favorable to those companies relating to Medicare payments.

The stock surge was prompted by an email sent by a Washington-based policy-research firm that predicted the change for its Wall Street clients. That alert, in turn, was based in part on information provided to the firm by a former congressional health-care aide turned lobbyist, according to emails reviewed by the Journal.

The activity surrounding the federal grand jury wasn’t previously known and signals that prosecutors haven’t given up on the matter.

An expanded version of this report appears on WSJ.com.

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