ST. PAUL - Minnesota's online student proficiency testing was suspended Wednesday for the second time this year after test provider Pearson experienced another cyber attack.

Brenda Cassellius, state education commissioner, said Pearson reported a "distributed denial of service attack" as students tried to take the science portion of the Minnesota Comprehensive Assessments.

Pearson told state leaders an outside source was trying to disrupt the testing system, leading to serious testing delays for students.

"It is simply unacceptable and unfair to subject students and teachers to this kind of uncertainty in a high-stakes testing environment," said Cassellius. "After the April 21 suspension, Pearson added additional security measures to prevent this type of disruption. Given the need to suspend testing today, I have questions about Pearson's ability to follow through on their assurances."

Pearson told state education leaders that Wednesday's cyber attack was larger and more sophisticated than one April 21.

Districts across the state reported problems logging into Pearson's testing system and slow delivery of test questions. Some educators said students were unable to submit completed tests to be scored.

Each year, students from third grade to high school take the MCAs in math, reading and science. About a million assessments are taken each testing season.

State education leaders have said many students completed online assessments without trouble.

Computer problems and a reported cyber attack slowed test-taking to a crawl on at least three days in April, leading the state to suspend exams for a day. The MCA testing window when students must complete the tests was also expanded by two days.

Minnesota has a three-year, $38 million contract with Pearson to provide online proficiency tests. The international firm is one of the largest U.S. test providers.

Minnesota education leaders planned to notify local school officials later today on plans for resuming testing.

Josh Verges contributed to this report.