Every one of the 31 Dallas County sheriff's deputies who took a sergeant's promotional exam last week failed, NBC 5 Investigates has learned.

The average score was 55 percent and the highest was 67 percent, according to a posting on the sheriff's website. Seventy percent is needed to pass.

Carmen Castro, a spokeswoman for Dallas County Sheriff Lupe Valdez, said Friday morning she was not aware of the results but would look into them.

In an email Friday afternoon, she said she was checking with the Dallas County Human Resources Department, which administers the test. But she did not provide any further information.

The president of the Dallas County Sheriff's Association said the test was from a new company.

"From what I've heard it was not a fair test," DCSA President Chris Dyer said. "If the test was flawed, then a lot of these people are going to have to retest and all the effort they put into studying was wasted.”

Dyer said the failure rate leaves the sheriff’s office with too few supervisors.

“We have a lot of open positions,” he said. “This really disrupts our ability to supervise."

Just six months ago, NBC 5 Investigates reported on massive failures at the sheriff's training academy.

Seventy-five percent of last year's recruiting class failed the exam to become licensed officers - the worst score in Texas.

State overseers put the academy on probation.

In May, Sheriff Valdez told commissioners the recruits may have deliberately bombed the test because they were angry with the department.

"You think they might have done this on purpose?” asked Dallas County Judge Clay Jenkins.

Valdez said it was possible but she didn’t know for sure.

"You can't prove that,” she answered.

The association of deputies denied anyone tried to tank the test. All 17 who failed passed on their second try.

When reached Friday, Dallas County Human Resources assistant director Stephanie Lang said she was not authorized to discuss last week's test.