Federal authorities are investigating how $2.6 million was taken from the retirement accounts of 58 city of Chicago employees via fraudulent loans.

Nationwide Retirement Solutions, the company that administers the city’s deferred-compensation accounts, spotted a series of suspicious transactions involving about 91 of the 36,000 employee accounts, city officials said Saturday. The accounts are similar to 401(k) accounts.

The company alerted city workers who were victims of the hack and refunded the missing money to their accounts, officials said.

The hackers used employees’ personal information and set up web profiles that allowed them to take out loans from the retirement accounts, city officials said, though 33 of the hacked accounts showed no bogus loan activity.