Hackers have repeatedly penetrated the computer network of the company that runs the Nasdaq Stock Market during the past year, and federal investigators are trying to identify the perpetrators and their purpose, according to people familiar with the matter.

The exchange's trading platform—the part of the system that executes trades—wasn't compromised, these people said. However, it couldn't be determined which other parts of Nasdaq's computer network were accessed.
Investigators are considering a range of possible motives, including unlawful financial gain, theft of trade secrets and a national-security threat designed to damage the exchange.