Texas says that 13 football players have confirmed positive coronavirus test results or are presumed positive, according to Allen Hardin, the school's senior associate athletics director for sports medicine and performance.

Texas says all 13 players are now self-isolating and 10 more identified through contact tracing are in self-quarantine but asymptomatic at this time.

In addition, four players have tested positive for the COVID-19 antibody. The Centers for Disease Control says it does not know yet if antibodies can prevent a second infection or, if so, how long protection might last.

The 13 positives includes two players who tested positive last week when 58 players initially returned to campus.

According to Big 12 Conference guidelines, the Longhorns were able to begin voluntary workouts on Monday.

Kansas State officials reported Wednesday night that eight Wildcats athletes have tested positive for COVID-19 out of 130 tested.

The NCAA on Wednesday approved plans for an extended preseason to give teams more time to hold walk-throughs. With the coronavirus pandemic wiping out spring practice for most teams, along with the usual required summer activities for players such as weight training and film study, schools want to make up for lost preparation time.

The Associated Press contributed to this report.