CHICAGO (From news reports) -- A hack targeting business communications company RR Donnelley was behind recent service outages at Fidelity and may have caused a six-day outage on Vanguard's website, according to news reports.

RR Donnelley disclosed on December 27 that it identified a "systems intrusion in its technical environment."

Around the time of the hack, Vanguard customers went on social media to complain about problems accessing account statements, trade confirmations, tax and other forms, as reported.

Last week, a Vanguard spokesperson said that the problems were due to a third-party service provider, without naming the company, and that the issue delayed "[c]ertain mailings and requests for checks by mail."

While Vanguard declined comment about which service provider was behind the delays, the firm's mail and check processing are handled by RR Donnelley, according to a former Vanguard technology employee who requested anonymity.

Fidelity, meanwhile, posted a service alert on its website that the RR Donnelley hack impacted Fidelity's printed communications services.

A Fidelity spokesperson told the publication in an email that the company is using "alternate vendors and procedures," without disclosing the vendors.

The spokesperson also confirmed that most of its communications to workplace clients and their employees are digital and weren't affected by the hack.

Andrew Stoltmann, a lawyer with Stoltmann Law Firm and a former president of Public Investor Arbitration Bar Association, says that service outages could attract regulatory scrutiny, according to Ignites.

And Amy Lynch, founder and president of FrontLine Compliance, says that if broker-dealers' monthly account statements were affected by the hack, that could lead to regulatory attention because of a Financial Industry Regulatory Authority rule requiring them to confirm when trades are executed, according to the publication.