Washington (CNN) -- At least one company and the National Republican Senatorial Committee have been issued subpoenas requesting documents in a federal grand jury investigation related to Sen. John Ensign, R-Nevada.
The Senate Ethics Committee also is investigating Ensign, who has been dogged by allegations of improper conduct stemming from an affair with the wife of a former aide. An Ethics Committee spokesman has declined to comment on precisely what its members are looking into.
A spokesman for the NRSC, which works to elect Republicans to the U.S. Senate, said the organization had received a subpoena related to Ensign.
Another NRSC official, general counsel Sean Cairncross, would not confirm that the subject of the subpoena was Ensign, but he noted the 2008 election cycle was the time when Ensign chaired the committee.
"NRSC has responded appropriately to questions concerning matters related to the 2008 election cycle timeframe," Cairncross said.
The U.S. Attorney's office had no immediate comment. Neither did Ensign's staff.
A source from one of the companies linked to the probe into Ensign's business dealings told CNN that the company had been issued a subpoena to produce documents to a federal grand jury.
The subpoena was for documents only, and no company officials have been asked to testify, according to the source, who spoke on condition of anonymity because that person is not authorized to speak on the record. The grand jury is scheduled to meet March 31 in Washington, the source said.
The source also said the company will comply with the request and has received no information that it is a target of any criminal prosecution.
Earlier, CNN affiliate KLAS-TV of Las Vegas, Nevada, posted online what it describes as a grand jury subpoena asking for documents related to Ensign.
Ensign told CNN in January that he would cooperate with any investigations that may be under way.
"We have said we will cooperate with any investigations," he said in January, "but at this moment, I'm just going to focus on being the best senator that I can be for my state."
KLAS reported Wednesday that subpoenas have been issued to local businessmen who may have dealt with Ensign and his Senate staff since January 2008. The station said an FBI agent and Justice Department prosecutor served subpoenas March 8 in Nevada, requesting documents on Ensign and current and former top aides.
The two-term Republican admitted in June to an extramarital affair with Cindy Hampton, his onetime campaign treasurer. She is the wife of top aide Doug Hampton.
The document that KLAS posted online asks for "any documents relating to or regarding" Cindy and Doug Hampton as well as John Lopez and Michael and Lindsey Slanker.
Lopez was the senator's chief of staff. The Slankers were political advisers who run the consulting firm November Inc. The document on the television station's Web site also seeks information about November Inc.
Ensign and his family were longtime friends with the Hamptons. Doug Hampton has given interviews stating his family has received money and employment offers from Ensign after he and his wife left the Senate staff in April 2008. Ensign admitted his parents gave the Hamptons $96,000 but said the money was a gift, not an effort to suppress word of the affair.
"Each gift was limited to $12,000," said a statement from Ensign's lawyer in July. "The payments were made as gifts, accepted as gifts and complied with tax rules governing gifts." Under U.S. tax laws, gifts of up to $12,000 are tax-exempt.
According to the statement, Ensign's parents learned of the affair from their son and decided to make the gifts "out of concern for the well-being of long-time family friends during a difficult time."
Before the senator's admission of an affair, he was considered a rising political star and possible Republican presidential candidate for 2012.