Tennessee’s student tests were derailed on Tuesday for a second straight day, this time by what state education officials say might have been “a deliberate attack.”

Unlike on Monday, when many students could not log on to the online TNReady exam, testing began smoothly for most districts — then started experiencing trouble within an hour.

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“Things were going OK, and then our students started having problems,” said James Evans, a spokesman for Rutherford County Schools. “Either they couldn’t log in, or they were not able to upload information when they were done, or the test just stopped and knocked them out of the system.”

The explanation, according to state Education Commissioner Candice McQueen: “It appears Questar’s data center may have experienced a deliberate attack this morning based on the way traffic is presenting itself,” she said in a morning email to school directors.

Tennessee hired the company Questar on a $30 million-a-year contract to run the testing program. Company officials said they were investigating the disruption, which they said they learned about at around 8:45 a.m. CST when students could not log on or submit their tests. But a Questar executive pointed to outside factors as the cause.

“Initial findings indicate it is external to our online delivery platform,” said Brad Baumgartner, Questar’s chief operating officer. “We are working with our hosting vendor to determine the root cause and have taken necessary measures to allow students to resume successful testing. At this time, testing has resumed.”

By noon, after a system “reset” by Questar, the state reported that testing had resumed and urged districts to pick up where they left off. McQueen tweeted that 22,000 students had successfully completed online testing so far and thanked districts for their flexibility and patience.

But many districts already had suspended testing for the day, including ones in Knoxville, Chattanooga, Bartlett, Collierville, Germantown, and Tullahoma.

“Our plan is to begin testing again tomorrow, once we receive information from the state that issue has been resolved,” said Carly Harrington, a spokeswoman for Knox County Schools.

The state’s largest school system left the decision up to its high school principals.

“We’ve told our schools that if you’re able to keep going to keep going,” said Superintendent Dorsey Hopson of Shelby County Schools.

State and Questar officials did not immediately respond to questions about who might have attacked the system. But in her letter to school directors, McQueen expressed concern that disruptions could continue in the future.

“The attacker may take these same steps again,” she wrote. “Questar is actively working on further reinforcements, including notifying authorities.”

She added: “To our knowledge, no student data has been compromised.”

McQueen ordered a three-day extension of TNReady’s online testing window to May 9. But the disruption to students, classes, and school schedules already was sweeping.

“Any time you have parents preparing, teachers preparing, students preparing and you get ready for game day and you start the game and then the game is called off, and then you have to start and stop, all of those things have an impact on your kids’ ability to do their best,” Hopson told Chalkbeat.

The second day of interruptions also created a logistical nightmare for teachers, said Leigh Ann Skaggs, a high school teacher in Chester County.

“Teachers have had to create lesson plans last minute, figure out how to continue to stretch out plans to prepare students for a test we aren’t sure they’re going to take, and hype them up for a test that may not happen at this point,” she said. “We are in a holding pattern to see what happens next. I’m not holding my breath.”

One Bartlett parent echoed a sentiment many educators have felt as Tennessee’s transition to a new test and online system have been fraught with problems. Sweeping technical problems two years ago caused the state to cancel many tests for the year — and fire its old test vendor before hiring Questar as a replacement.

“This is a very high-stakes test that impacts student report cards, teacher evaluations, and employment, and even determines soon-to-be letter grades for schools and districts,” said Jennifer Proseus. “Why do these faulty tests — that parents and teachers are forbidden from seeing — hold so much power?”

This story has been updated.