A Jacksonville-based church organization is trying to get back more than $700,000 stolen by cyber thieves.

The nation’s cybersecurity agency warned of a “grave” risk to government and private networks. The hack compromised federal agencies and “critical infrastructure” in a sophisticated attack that was hard to detect and will be difficult to undo, the Cybersecurity and Infrastructure Security Agency said in an unusual warning message. The agency previously said the perpetrators had used network management software from Texas-based SolarWinds to infiltrate computer networks. The cybersecurity agencies of Britain and Ireland issued similar alerts. Members of Congress said they feared that taxpayers’ personal information could have been exposed because the IRS is part of Treasury, which used SolarWinds software.