She added that the FCC is cooperating with the House Energy and Commerce Committee's probe of LightSquared.
"In the LightSquared matter, the commission is following longstanding practice, consistent with Congress’s own guidance with respect to requests from individual members. We have repeatedly made this clear," Sun said.
FCC officials have noted that agencies usually only respond to inquiries from lawmakers who serve on committees with jurisdiction over them. Grassley is the top Republican on the Judiciary Committee, but does not serve on any panels that oversee the FCC.
Sun also dismissed Grassley's insinuation that Josh Gottheimer, senior counselor to the FCC chairman, was biased toward LightSquared.
In a memo to reporters on Wednesday, Grassley's office noted that Gottheimer previously worked for a public-relations firm that now works for LightSquared.
Grassley said the commission was ignoring his requests to question Gottheimer over his involvement with LightSquared.
Sun said Gottheimer joined the FCC in July 2010, months after the commission granted LightSquared its conditional waiver. She added that an ethics review concluded that he never worked for LightSquared when he was at the public-relations firm Burson-Marsteller.
LightSquared planned to launch a nationwide 4G wireless network, but tests showed its signals would interfere with GPS devices.
After a review concluded there was no feasible way to fix the interference problem, the FCC pulled the company's waiver and has now moved to block the launch of its network.
Grassley is questioning why the FCC allowed LightSquared to get as far as it did in the regulatory process.
He has pledged to block President Obama's two FCC nominees unless the agency releases internal documents related to its review of the company.
“Throughout my career, I’ve received documents from agencies across the federal government in response to my oversight inquiries," Grassley told The Hill in response to the FCC statement Thursday. "It’s rare to see an agency go to such extremes to avoid a response to a simple request unless the agency has something to hide. In October, the FCC chairman pledged to make staff members available to speak with me. I took him at his word – my mistake.”
Other Republicans, including Rep. Michele BachmannMichele Marie BachmannBoehner says he voted for Trump, didn't push back on election claims because he's retired Boehner: Trump 'stepped all over their loyalty' by lying to followers Boehner finally calls it as he sees it MORE (Minn.), have accused the White House and the FCC of favoring LightSquared because of political connections, pointing to billionaire investor Phil Falcone.
Falcone, who has donated thousands of dollars to both Democrats and Republicans in recent years, says he is a registered Republican.
The White House and the FCC have denied showing LightSquared any inappropriate favoritism although expanding broadband Internet access is a top priority of both agencies.
--Updated at 4:17 p.m.