In the next few weeks, letters will again be sent to Christian television ministries that have not responded or raised concerns about the probe, which is being conducted by the Senate Finance Committee led by Grassley.
Only two of the six ministries under investigation - Joyce Meyer Ministries and Kenneth Copeland Ministries - turned over their documents last month and only Meyer has pledged full cooperation.
The probe began in November after ministry watchdogs alleged opulent spending and possible abuse of their nonprofit status. Besides Meyer and Copeland, Grassley sent letters to Paula and Randy White, Creflo Dollar, Eddie Long and Benny Hinn - all of whom are popular televangelists whose ministries each collect tens of millions of dollars in donations each year.
Grassley requested cooperation from the ministries, asking that they provide financial records as well as responses to a wide range of questions regarding their personal and organisational finances.
Paula and Randy White and Benny Hinn have been in contact with Grassley's office but have yet to give a clear response on whether they will cooperate.
The other two ministers, Creflo Dollar and Eddie Long, who have raised questions about the committee overstepping its authority and its attack on religious freedom, said they will not comply. They also made it clear that they have been following IRS rules and contend that the IRS, not the Senate committee, is the proper forum for investigating complaints.
Jay Sekulow, a Washington lawyer in evangelical and Republican circles who heads the American Center for Law and Justice, has given advice to the ministries, according to two anonymous sources, as reported by The Associated Press. And while the ACLJ is concerned about the constitutional issues raised by the inquiry, it will not be making any appearances on behalf of any of the ministries before the committee, a statement by the centre read.
"It's been extremely rare for tax-exempt groups to decline to cooperate with his requests for information," Jill Gerber, spokeswoman for Grassley's finance committee, told AP. "Tax-exempt groups usually work to answer questions from a leader of the Senate committee that sets tax policy. Senator Grassley hopes that these groups will be similarly cooperative with this inquiry."
The follow-up letters will describe Congress' authority and duty to investigate and evaluate laws over which it has legislative authority - in this case, tax-exempt policy, according to AP.
The Senate probe has garnered support from several Christian groups as well as opposition from others who fear the investigation may delve improperly into theological questions. The six televangelists preach what critics call the "prosperity gospel", which teaches wealth as a sign of God's blessing.