Six Dane County supervisors are requesting an external audit of Dane County’s 911 Center after hearing the center failed to return a phone call from slain University of Wisconsin junior Brittany Zimmermann’s cell phone the day she was killed.

The supervisors have sent Dane County Board of Supervisors Chair Scott McDonell a letter requesting a meeting with several officials involved in the case to further investigate the process and learn more about the case.

911 Center Director Joe Norwick has declined to provide details on the call due to ongoing investigations, but said a dispatcher received a call from Zimmermann’s phone, got busy with other calls and failed to return it, breaking 911 policy.

In light of the incident, Dane County Executive Kathleen Falk has written letters of apology to Zimmermann’s family to be sent this week, and McDonell apologized Monday afternoon.

Madison Police Department Chief Noble Wray said last week the content of the call indicated an officer should have been sent to investigate the situation.

Officials from the 911 Center have said there is an ongoing internal investigation to determine what happened with the 911 call placed from Zimmermann’s phone, but the supervisors are wary about the process, which has taken more than a month.

Supervisor David Wiganowsky, who signed the letter, said an internal audit of the 911 Center “doesn’t make much sense” and added he would rather have an external audit put in place to avoid a similar situation in the future.

“Some awful accusations have been made, and they should either be confirmed or denied,” Wiganowsky said.

Supervisor Wyndham Manning, District 5, who did not sign the letter, said a meeting is scheduled for Thursday, and Norwick, Wray, Falk and Dane County Sheriff Dave Mahoney have been asked to attend.

He added the board has not been briefed on the case, and this week’s meeting would be an opportunity to hear firsthand about case details not made available to the public.

“Obviously, mistakes were made, and we need to figure out where those mistakes were made,” Manning said.

If the meeting takes place, it would be open to supervisors only.

McDonell did not immediately return a phone call seeking comment Monday.