We were reassured in mid-June when the House ethics committee announced — after being prodded by the full House — that it is investigating allegations involving the PMA Group, but now there’s reason to question the way the probe is being conducted.

As Roll Call reported Tuesday, the inquiry is being conducted under Rule 18(a), a procedure allowing for maximum secrecy by the committee — and no subpoenas, the most effective method of garnering evidence.

Rule 18(a) investigations, which can be initiated by just the committee’s chairwoman and ranking member, are the most common form conducted by the committee. They allow for voluntary interviews with Members and staff and often conclude with no disclosure.

But that won’t do for the PMA scandal, which has the potential — and we stipulate, only the potential — of being the disaster for Democrats that the Abramoff scandal was for Republicans prior to the 2006 elections.

As most everyone knows, the now-defunct lobbying giant funneled more than $5 million in campaign contributions to eight influential Democratic Members over a decade while some of those lawmakers secured tens of millions in earmarks for PMA’s clients.

In 2006, Republican lobbyist Jack Abramoff pleaded guilty to federal charges of defrauding American Indian tribes and corrupting federal officials. Two executive branch officials also were convicted, along with Republican Rep. Bob Ney (Ohio) and nine lobbyists and former Congressional aides.

The scandal also enmeshed several other Members whose reputations were damaged and cloaked Republican management of the House in an aura of corruption — but most of it was never investigated by the Committee on Standards of Official Conduct.

By initiating its probe of PMA, the committee certainly is ahead of that dismal record, even though its announcement came months after an FBI raid on the firm, after an avalanche of investigative journalism on its ties to Members and after the whole House voted to call on the committee to reveal whether an investigation was under way.

But the public needs to be assured that its investigation is penetrating and thorough — and, especially, that no favoritism is being shown to powerful Members such as Rep. John Murtha (D-Pa.), chairman of the Appropriations Subcommittee on Defense and a close ally of Speaker Nancy Pelosi (D-Calif.).

Under Rule 18(a), the committee does not need to reveal what Members it might be investigating or what House rule violation is involved. And, indeed, it has not.

Such revelations would be traditional if the full committee voted to create an investigative subcommittee or to issue subpoenas. We assume that the committee’s staff members will be working over the August recess. When Congress returns, we hope the full committee will act to demonstrate that a complete probe is under way — one that will either absolve Members of suspicions of corruption or recommend appropriate punishment.