Frustrated by replies to its initial set of subpoenas, the Senate Indian Affairs Committee on Wednesday approved eight more in its ongoing investigation of multimillion-dollar lobbying and public-relations contracts signed by a handful of tribes.

Aides said the latest batch of subpoenas were designed to pull the probe into its final stages, with the hope of holding a hearing on the matter in mid-September.

Acting in a closed, executive session, the panel did not indicate who was named in the subpoenas. The top targets of the probe have been former lobbyist Jack Abramoff and Mike Scanlon, the public relations executive who was once Rep. Tom DeLay’s (R-Texas) top media adviser.

After the first round of subpoenas were issues five weeks ago, Chairman Ben Nighthorse Campbell (R-Colo.) and ranking member Daniel Inouye (D-Hawaii) indicated that Abramoff and Scanlon, as well as the firms they worked for, should expect a legal demand to provide information into the probe. It also appeared likely that some of the tribes would be compelled to turn over documents.

The committee has been examining more than $45 million in lobbying and PR fees several tribes paid to Abramoff and Scanlon over a three-year period. Questions have arisen about whether such rates were overly exorbitant, and whether they came from tribal funds dedicated to other matters, such as education accounts.

Abramoff and Scanlon have denied any wrongdoing, asserting that their fees are par for the course in the multibillion-dollar Indian gaming industry.

Several federal agencies are looking into the matter as well. But Senate aides said the committee has not been asked to stand down in its investigation, even as the two parallel probes seek out the same documents and interviews with the same principals.

Sen. John McCain (Ariz.), the second-ranking Republican on Indian Affairs, said the committee has received only a “few” replies to its first round of subpoenas, but said he was not yet upset with the responses because the requests were so broad that it may take time for everyone to comply with the requests.

“They’re pretty far-reaching,” he said. “I accuse no one yet of obfuscation or delay.”

In addition to issuing the new batch of subpoenas, the committee granted Campbell and Inouye the power to issue more subpoenas without a formal committee meeting. That was done, aides said, because of the chance that something might happen in the next seven weeks, while Congress is in recess, that requires a quick subpoena to be issued.

McCain, however, expects Wednesday’s batch of subpoenas to be the last that will be necessary. “I don’t anticipate needing any more before September,” he said.