The Federal Reserve detected more than 50 breaches of its computer systems from 2011 to 2015, Reuters reported, adding to signs that the central bank may be vulnerable to hackers or spies.

Hacking attempts were cited in 140 of 310 total reports provided by the Fed, and four incidents in 2012 were called acts of “espionage,” Reuters said in an article Wednesday based on Fed records obtained through a Freedom of Information Act request. At least two of the espionage cases resulted in information being disclosed. The Fed's national cybersecurity team identified 51 cases of “information disclosure,” Reuters reported.

The documents, later obtained by Bloomberg, span 2,239 pages and are heavily redacted. The records don't identify hackers or say whether sensitive information was accessed or money was stolen, Reuters said.