The Air Force Personnel Center said Monday that 5,489 enlisted airmen were ineligible for consideration for promotion to technical sergeant because they didn't finish the required online professional military education course.

The Air Force announced July 11 that 25,552 staff sergeants were eligible for promotion to E-6 this year — a sharp decline from the 33,569 who were eligible for the 2016 tech sergeant promotion cycle. The Air Force did not say last week what caused the steep decline.

In an email to Air Force Times, AFPC spokesman Mike Dickerson on Monday confirmed that much of the decline was caused by airmen's failure to complete PME.

Had those airmen finished their online education and been able to compete for promotion, Dickerson said, the pool of eligible airmen would have been around 31,000. A pool of roughly 31,000 E-6-eligible airmen still would have been the lowest in decades, but would have been closer to the average in recent years.

Dickerson also said that the higher promotion rates in recent years, as the Air Force has sought to rebuild its ranks after the steep drawdown in fiscal 2014 and part of fiscal 2015, also could have contributed to a smaller eligibility pool this year.

"While this [PME-based ineligibility] was a large contributor to the smaller eligibility pool, changes from year to year are not unusual," Dickerson said. "Our mid-grade enlisted ranks, mainly E-4 through E-7, contained the majority of the 2014/15 enlisted reductions. As we've worked to regrow the force, we've utilized higher promotion rates in these ranks to help fill key vacancies. When larger numbers of airmen in a particular rank and year are promoted, the size of the following year's eligibility pool can sometimes be reduced."

Other airmen may not have been able to be considered for promotion this time because they were facing a pending separation or an Article 15 proceeding, they were deployed, or they did not have their most recent static closeout date evaluation updated in time, Dickerson said. Many airmen who miss the promotion cycle can be considered for supplemental promotion when they meet all their requirements.

About 9,300 airmen were left ineligible for promotion or re-enlistment after missing the deadline to finish their PME by the end of 2016. In March, the Air Force partially reversed course and said those airmen would be allowed to re-enlist, although they would remain ineligible for promotion.