Following warnings earlier this month, six major American banks were hit in a wave of computer attacks last week that caused web blackouts and delays in online banking, reports The New York Times.

Read: U.S. banks may face cyber attacks

Customers of Bank of America, JPMorgan Chase, Citigroup, U.S. Bank, Wells Fargo and PNC couldn’t access their accounts or pay bills online, and were upset the banks didn’t explain why the problems were occurring at the time.

The attacks weren’t technically sophisticated and didn’t affect any of their computer networks, however.