Financial regulators have built an elaborate computer apparatus to scan stock markets for signs of insider trading, but the radar isn’t pointed at Congress.

Instead, it is designed to detect more conventional forms of insider trading — a corporate lawyer acting on advance word of a merger or an accountant leaking details about a company’s earnings. A senator who calls his broker after receiving a closed-door briefing might not even raise a blip.

That’s because the radar was built for the brokerage industry to spot the kind of abuses that were historically of concern: trading on inside information related to developments in the private sector. The apparatus is run by an industry self-policing group, which notifies the Securities and Exchange Commission of suspicious activity.

Congress is considering legislation that would make clear that lawmakers cannot cash in on information they acquire through their privileged positions — for example, inside knowledge of legislative negotiations. The House and Senate have both passed versions of the bill, and a conference committee will meet this month or in early March to work out differences.

But the absence of a law may not be the only reason the SEC has never charged a member of Congress with insider trading. Not only are there blind spots in monitoring, but also it could be awkward for the agency to investigate lawmakers, especially those who oversee the SEC and control its funding.

The Washington Post reported this week that the Office of Congressional Ethics is probing whether Rep. Spencer Bachus (R-Ala.), chairman of the House Financial Services Committee, violated insider-trading laws.

“I have fully abided by the rules governing Members of Congress and look forward to the full exoneration this process will provide,” Bachus said in a statement Friday.

In keeping with agency policy about pending investigations, SEC spokesman John Nester would not say whether the SEC is looking at Bachus’s trades.

The matter puts the SEC in a potentially uncomfortable position. Bachus is one of the agency’s primary overseers in Congress, and the committee he chairs holds power over the SEC’s budget.

“He can make life very unpleasant for the agency,” former SEC chairman Arthur Levitt said.

Another former SEC official said the agency has investigated members of Congress for possible insider trading in the past but has decided not to bring charges against them. The official said it was unclear why the SEC refrained — for example, whether there was a lack of proof, or whether existing laws made it difficult to prosecute members of Congress.

A third SEC veteran, former chairman Harvey L. Pitt, said that without the kind of clarification offered by the new bill, the SEC would be unlikely to investigate a member of Congress in connection with insider trading.

“The prevailing feeling is that in the absence of special legislation, this would not be something the SEC could go after,” Pitt said in an interview.

In December, the House Financial Services Committee convened a hearing on the new bill — the Stop Trading on Congressional Knowledge Act, or Stock Act — with Bachus presiding.

The public has “an absolute right to demand that people who they elect to represent them in Congress conduct themselves according to the highest ethical standards and do not seek to profit from their positions,” Bachus said as he opened the hearing.

In testimony that day, SEC enforcement director Robert Khuzami said that, under current law, pursuing insider-trading charges against a member of Congress could involve unusual complexities.

“There is no reason why trading by members of Congress or their staff members would be considered ‘exempt’ from the federal securities laws, including the insider-trading prohibitions,” Khuzami said, but he added that applying those to members of Congress “is without direct precedent and may present some unique issues.”

For example, it is illegal for people to trade on confidential information when they have a duty to their employer not to misuse the information. But there do not appear to be court precedents on whether lawmakers are under a similar obligation, Khuzami said.

In addition, even if insiders do not trade on confidential information themselves, they can be prosecuted for tipping off others who do. But if members of Congress or their staff members are the ones passing the tips, their conduct could fall within the realm of Congress’s constitutionally protected speech, Khuzami said.

Whether politics play a role is a different question. In a statement Friday, Khuzami said no.

“We go where the evidence leads, pure and simple,” he said.

Historically, the evidence presented in insider-trading cases has generally led to the business world, and that’s where regulators have aimed one of their main investigative tools.

The radar system, known as “market surveillance,” monitors the purchase and sale of stocks — billions of shares a day — and is administered by FINRA, the Financial Industry Regulatory Authority, a self-regulatory organization for the brokerage industry.

The system, for the most part, has been programmed to focus on trading around developments that can move the stocks of individual companies, a regulator said, speaking on condition of anonymity to describe investigative tools. An example: a biotech company winning approval to market a new drug.

The surveillance “tends to be very company-specific and event-specific” and “tends to follow the existing case law,” the regulator said.

If the event in question — say, a private meeting on Capitol Hill — is not on the radar, the related trades would not necessarily attract attention.

“I think there’d have to be some external reason . . . to be suspicious about the trade,” the regulator said.

When FINRA spots suspicious trades placed by brokerages, it can ask the brokerages for more information, including the identities of the customers placing the trades, and it can pass its findings to the SEC.

FINRA’s market surveillance is not the only source of leads for law enforcement. There are others, both human and technological. Another self-regulatory group monitors options trading.

At the December hearing, Khuzami said the SEC is expanding its own capabilities. An SEC project “still in its early stages of development” can identify relationships among multiple traders and recognize suspicious patterns, he said.

“Our surveillance capabilities are not limited to those of the self-regulatory organizations, nor are they confined to analysis of events,” the SEC spokesman, Nester, said by e-mail. He declined to be more specific.

One lawmaker the SEC investigated for possible insider trading was former Senate majority leader Bill Frist (R-Tenn.). However, the case did not appear to involve information Frist obtained in his role as a legislator.

At issue was Frist’s sale of stock in HCA, a hospital company founded by his family, days before the company disclosed disappointing financial news and the stock price plunged. A federal subpoena was carefully crafted to focus on Frist’s private affairs and avoid running afoul of any congressional privilege, The Post reported at the time.

In 2007, the SEC and the Justice Department closed the highly publicized case without filing any charges.

Staff writer David S. Fallis contributed to this report.