Hackers who infiltrated the Nasdaq’s computer systems installed malicious software on the exchange’s computers, allowing them to spy on directors of publicly held companies, according sources familiar with an investigation.

Nasdaq disclosed in February that its systems were hacked last year.

That sparked an investigation involving the FBI and National Security Agency that is ongoing.

Nasdaq’s trading platforms were not compromised, the exchange said after the hacking, but an Internet-based software program known as Directors Desk was attacked.

Investigators said that hackers were able to spy on the computer systems of “scores” of directors who logged on before the malicious software was removed.