State IT officials say they have concluded their investigation of alleged "robocalling" that jammed the MNsure call center last month, and sent the case to the FBI for review for potential criminal activity.

At the start of open enrollment on Nov. 1, the MNsure call center was flooded with calls, causing lengthy waits for health insurance shoppers trying to quickly buy 2017 coverage.

DFL Gov. Mark Dayton blamed the problem that day on automated calling systems that most often are used in ­telemarketing.

"Automated calling is one possible explanation for the large call volumes seen on the first day of open enrollment this year," the state's IT services division said this week in a statement. Officials said they had turned "research over to the FBI's Computer Crimes Task Force for further investigation and review for potential criminal activity."

An FBI spokesman said he could neither confirm nor deny the investigation.

"One of the challenges of forensics investigations of call records is that it is virtually impossible to determine the true identities of callers, due to spoofing and other mechanisms to shield identities," said Chris Buse, the state's chief information security officer, in a statement.

"It can be very difficult to attribute these types of ­incidents to the actual perpetrators, because the underlying call records are subject to manipulation," Buse said.

MNsure is a government-run health insurance exchange that is part of the federal Affordable Care Act. It's an option for the roughly 250,000 state residents who buy coverage in the individual market, typically because they are self-employed or don't get health insurance benefits from their employer or a government program.

Open enrollment at MNsure continues through Jan. 31. The MNsure call center has not seen a repeat of problems from opening day, when state officials say there were about 50,000 call attempts by 9 a.m. That tally compared with just 1,600 by the same time the following morning.

The vast majority of callers on opening day did not take any action beyond selecting English as their language option, said Allison O'Toole, the MNsure chief executive, during a news conference at the time.

"Early on Tuesday [Nov. 1], something seems to have happened with call volume that was unusual and suspicious," O'Toole said in early November.

Call center wait times were a critical issue at MNsure during its launch in 2013, when thousands phoned for help after running into problems with the exchange's balky website. The website has improved since then, and wait times generally have been much shorter.

Even so, some users have complained again this year if they need help from a call center specialist. While calls typically are answered quickly by workers who can handle minor issues, people put on hold for a specialist can be on the phone much longer.

When asked about this distinction in November, a MNsure spokeswoman said via e-mail: "The average call wait time for the initial call is under one minute. ... The average call wait time for those more complex secondary calls is about 20 minutes."

"That's not what we prefer," she wrote, "but fortunately most people don't have to wait anywhere near that long."

The opening day of the enrollment period also saw a key portion of the MNsure website go down for about 30 minutes at midday. State officials said the outage, which affected 70 state websites, was caused by a configuration issue between software and hardware components.

The configuration issue stemmed from a scheduled change to the "virtualized server environment" that should have not have affected websites during normal business hours, state officials said.

"We found that a particular piece of software that manages our web content management system had behaved differently than anticipated ruing this configuration changes," IT officials said in a statement. "This caused the system to reboot itself."

Within 30 minutes of the problem on Nov. 1, state officials said they rebooted the web content management system and websites returned. An investigation after the incident let the state "implement new processes to mitigate and prevent future occurrences," officials said in a statement.

Twitter: @chrissnowbeck