If anyone could get Robert Diamond, the former chief executive officer of Barclays Plc, to explain the Libor-fixing scandal to a British parliamentary panel on Wednesday, it was Andrea Leadsom, a Conservative member and a former Barclays banker. Although their interchange generated some heat, it didn’t yield enough light.

Yes, Diamond was contrite, repeatedly condemning the conduct of the 14 traders involved. Yes, he lost his job. But not once did he let Leadsom and her fellow MPs delve into the bank’s internal operations, including its compliance process, risk-management procedures and managerial oversight to understand how Barclays staff could have falsified interest rates used to price $800 trillion in derivatives and corporate and consumer loans, with no one noticing.

Without these details, regulators and lawmakers will remain at sea over how to prevent such behavior at Barclays and the dozen or so other banks about to get pulled into the dragnet. Now that U.K. lawmakers have voted to conduct a parliamentary probe, here’s what they should ask Diamond:

-- “Where were the controls?” In four short words, the MPs should have demanded to know why various checks and balances, which all banks must have, failed at every level. How could bankers and traders, for almost five years, get away with rigging Libor submissions to make sure their derivative bets would pay off? As Diamond would have us believe, knowledge of the rate-fixing never went beyond trading desk supervisors. If that’s true, it reveals a lack of internal controls that borders on malfeasance.

It’s especially curious that Diamond, who more than most understood the culture of traders, having spent most of his long banking career either on trading desks or overseeing them, didn’t more closely monitor those at Barclays.

For example, Barclays has a compliance operation whose staff has the ability to check books and records, including e-mails and phone calls, most of which are archived for this very purpose. They can and often make surprise spot checks. What exactly did Diamond ask them to do? Other employees are similarly empowered to look behind traders’ books to avoid losses and keep liabilities in check. These include the risk-management staff, outside auditors and legal counsel. Did any of them conduct routine, or even occasional, inspections?

-- “When did you first learn about the falsifications?” Actually, we know the answer to this one. Diamond claimed at the hearing that he learned only in mid-June about e-mails showing that his bank had made false submissions on interest rates used to set the London interbank offered rate. (Libor is derived from banks’ own estimates of what it would cost to borrow from one another in various currencies and timeframes.) The e-mails, he said, made him “physically sick.”

We have to take him at his word, though his answer stretches credulity. The MPs could have asked more probing questions about a telephone conversation Diamond had with Paul Tucker, a senior Bank of England official, in October 2008, during the financial crisis. Diamond at the time wrote a note to himself that Tucker said it “did not always need to be the case that we appeared as high as we have recently,” referring to Barclays’ daily Libor submissions. Diamond said he thought Tucker was giving him a heads-up that the U.K. government might be thinking of nationalizing Barclays if it couldn’t fund itself. He didn’t think the central bank was telling him to fudge his numbers so the bank would look healthier.

Documents made public Tuesday, however, show that Jerry Del Missier, the then-president of Barclays’ investment bank concluded the opposite -- that an instruction had come down from the central bank to get quotes in line with those of other banks. The next day, the borrowing rates submitted by Barclays fell sharply. Here’s the question Diamond should have been compelled to answer: If you knew your Libor submissions were high enough to worry the government about Barclays’ viability, didn’t you check to see what your submissions were the next day, and every day thereafter? And when they suddenly dropped, didn’t you ask why?

-- “Why didn’t you examine Barclays’ numbers?” After the conversation with the Bank of England official, Diamond said he asked Barclays’ then-CEO to relay to the government that other banks were understating their borrowing costs. If he knew this, why didn’t Diamond ask to see the calculations his staff had been submitting? Or at the very least, why didn’t he ask the head of compliance about it?

-- “Do you think you should forfeit the 14.8 million pounds ($23 million) in future share awards, or give back some of the 120 million pounds ($186 million) in salary, benefits and bonuses you’ve received since 2005?” Diamond was asked about the share awards he’s due to receive once he leaves. He deflected the question by saying it was up to the board, not him. Considering the extent of what he seems to have missed, shouldn’t clawing back past compensation be on the table, too? In the end, that may be the only way regulators, lawmakers and taxpayers can ward off such behavior. In case the board needs help with the answer, it’s “yes.”

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Today’s highlights: the editors on the best outcome of Libya’s messy elections; Jonathan Weil on what else might be amiss at Barclays; Stephen L. Carter on the uselessness of the vice presidency; Noah Feldman explains why liberals should be happy with the Supreme Court; Enrique Krauze on democracy and new leadership in Mexico; Vali Nasr on the U.S. apology to Pakistan; William Pesek on economic development in the Mekong River region.

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