WASHINGTON (CNN) -- The Senate Ethics Committee issued a letter of admonition Thursday to New Mexico Republican Sen. Pete Domenici for his role in contacting the U.S. attorney in his state about a pending federal grand jury investigation into public corruption.

Sen. Pete Domenici, who suffers from a brain disease, is retiring after six terms in the Senate.

"The committee finds no substantial evidence to determine that you attempted to improperly influence an ongoing investigation," the letter said about his call to David Iglesias, then the U.S. attorney for the District of New Mexico.

Iglesias is one of eight U.S. attorneys fired by Bush administration Attorney General Alberto Gonzales, which led Democrats to allege the firings were politically motivated. Iglesias has said Domenici called him before the 2006 election to inquire about an ongoing corruption investigation involving Democrats, which he interpreted as political pressure. The controversy led to Gonzales' resignation.

"The committee does find that you should have known that a federal prosecutor receiving such a telephone call, coupled with an approaching election which may have turned on or been influenced by the prosecutor's actions in the corruption matter, created an appearance of impropriety that reflected unfavorably on the Senate."

In a public statement issued in March 2007 about the October 2006 call, Domenici wrote, "In retrospect, I regret making the call and I apologize. However, at no time in that conversation or any other conversation with Mr. Iglesias did I ever tell him what course of action I thought he should take on any legal matter. I have never pressured him nor threatened him in any way."

Thursday's letter from the committee represents a black mark for Domenici, 75, who is retiring after six terms in the Senate. He is suffering from a progressive brain disease.

Don't Miss Senate seeks federal inquiry of altered earmark

In a written statement, Domenici said Thursday, "I am gratified the Senate Ethics Committee has concluded its inquiry favorably without a formal adjudicatory review, confirming what I have always maintained: I did not attempt to improperly influence an ongoing investigation when I telephoned the former United States Attorney for the District of New Mexico.

"Nevertheless, as I said publicly last March when this complaint was originally filed, I regret the distraction this controversy has caused my colleagues, my staff, my family and, most importantly, my constituents. Now that this matter has concluded favorably, I am anxious to focus all of my time and attention on the business of the people of New Mexico." E-mail to a friend

All About Pete Domenici • New Mexico • U.S. Senate Select Committee on Ethics