Senators from both sides of the aisle are expressing concerns about the coverage map the FCC is planning to use to decide where to put more than $4.5 billion in rural broadband subsidies, and they want more time to challenge the agency's findings.

The FCC put out a map of areas eligible for Mobility Fund Phase II money over the next decade as part of its move to redirect wireless carrier subsidies where private capital was already at work for, as FCC chair Ajit Pai put it, "something far more useful: bringing 4G LTE service to rural Americans who don’t have it today."

Targeting subsidies to unserved areas is an FCC priority under Pai, as well as for carriers who are not looking to be overbuilt by government funds.

Pai also said the redirect would include a "robust challenge process to identify the areas most in need of service."

But a group of senators led by Sens. Roger Wicker (R-Miss.) and Maggie Hassan (D-N.H.) said the map is significantly flawed; the group wants the FCC to extend the window for that robust challenge by another 90 days.

In a letter to Pai the senators said the FCC map shows areas in their home states that are purportedly served by 4G LTE, when experience on the ground suggests otherwise.

Also signing on to the letter (reprinted below) were Sens. Brian Schatz (D-Hawaii), Roy Blunt (R-Mo.), Angus King (I-Maine), Cory Gardner (R-Colo.), Sherrod Brown (D-Ohio), Pat Roberts (R-Kan.), Jeanne Shaheen (D-N.H.), James Lankford (-Okla.), Ron Wyden (D-Ore.), Richard Shelby (R-Ala.), Tammy Baldwin (D-Wisc.), Cindy Hyde-Smith (R-Miss.), Patrick Leahy (D-Vt.), Ron Johnson (R-Wisc.), Amy Klobuchar (D-Minn.), Todd Young (R-Ind.), Thom Tillis (R-N.C.), Christopher Coons (D-Del.), Claire McCaskill (D-Mo.), Kamala Harris (D-Calif.), Tina Smith (D-Minn.), Tom Udall (D-N.M.), Catherine Cortez-Masto (D-Nev.), Tammy Duckworth (D-Ill.), Doug Jones (D-Ala.), Edward Markey (D-Mass.), and Deborah Fischer (D-Neb.).