The hackers behind one of the worst breaches in US history read and downloaded some Microsoft source code, but there’s no evidence they were able to access production servers or customer data, Microsoft said on Thursday. The software maker also said it found no evidence the hackers used the Microsoft compromise to attack customers.

Microsoft released those findings after completing an investigation begun in December, after learning its network had been compromised. The breach was part of a wide-ranging hack that compromised the distribution system for the widely used Orion network-management software from SolarWinds and pushed out malicious updates to Microsoft and roughly 18,000 other customers.