The fact that the Web-based service, called Directors Desk, was penetrated could lend credence to one theory that law-enforcement authorities investigating the matter are considering, namely that hackers may be aiming to extract nonpublic inside information that could be used illegally to gain a trading edge.

However, several people with knowledge of the probe say investigators haven’t reached even preliminary conclusions about motive and that other theories, including terrorism, theft or wire fraud, at this point remain possibilities.

The Wall Street Journal reported Friday night that outsiders had repeatedly penetrated the computer network of Nasdaq OMX Group during the past year. The exchange’s trading platform-the part of the system that executes trades-wasn’t compromised, these people said.

The company and people familiar with the investigation say as far as they can tell no information from Directors Desk, which is operated separately from Nasdaq’s trading platform, was taken or compromised. On Saturday, Nasdaq said that after the Journal’s report, it immediately decided, in consultation with authorities, to inform customers. Prior to that, the company said, the Justice Department had asked that it refrain from notifying customers until at the earliest Feb. 14 to facilitate the investigation. A spokesperson for the department declined to comment.

Nasdaq spokesman Frank DeMaria said Sunday that the company detected the security issue in October or November and reported it to the Securities and Exchange Commission, the Federal Bureau of Investigation and the Justice Department.

The files in question were removed and Nasdaq made modifications to the system as a deterrent, he said. He declined to give further details about changes Nasdaq made.

There were indications Sunday that the security issues affecting Nasdaq had engendered broader concerns.

NYSE Euronext also has a Web service for corporate-director communications called eGovDirect.com. NYSE notified customers Wednesday that it was shutting the site for technical reasons, and it remained down in light of those issues and broader security concerns, said a person familiar with the matter. On Sunday a message on the site said it was temporarily unavailable “due to an unscheduled maintenance activity.”

An NYSE spokesman said the company doesn’t comment on security matters. He said the eGovDirect system is operated separately from NYSE trading.

People familiar with the Nasdaq case say that while the specifics of that hacking aren’t particularly egregious in a world where corporate networks are attacked daily, the case has raised alarms in the government because of the potential implications of compromising Nasdaq, which runs one of the world’s most-important exchanges.