Federal prosecutors are investigating charges that Rep. Daniel J. Flood (D. Pa.) and a close friend of his tried to buy the silence of the congressman's onetime chief staff aide, who was under investigation for bribery.

The former aide, Stephen B. Elko, was convicted on the federal charges in Los Angeles last fall. He has been cooperating with federal authorities under a grant of immunity since then and is reported to have accused Flood of taking more than $100,000 in bribes over the years.

Elko also has implicated Flood and Washington attorney John L. (Roy) Ingoldsby in a scheme to obstruct justice in his case, according to sources familiar with that separate investigation.

Those charges center on arrangements that were allegedly made to provide Elko with income, and the payment of $25,000 in legal expences after he was forced to resign from Food's staff in June 1976 to head off a House Committee investigation of his activities, the sources said.

The U. S. attorney's office in Los Angeles also is examining Flood's testimony before a grand jury there and at Elko's trial for a possible perjury charge, the sources added.

Though he is not a criminal lawyer, Ingoldsby, a longtime personal friend of Flood's, was Elko's attorney for more than a year and a half before the aide's indictment.

He said in a recent interview that he did arrange for the unusual payment of Elko's legal expenses, using $15,003 from another close friend of Flood's and $10,000 of his own to pay trial costs.

"Sure I was helping Flood," Ingoldsby said. "Sure I went farther than I would have normally. But I did not obstruct justice . . . This was no Watergate hush-money. It was all handled by check."

In a more formal statement issued late yesterday, Ingoldsby said: "These were acts of compassion and generosity, which did not obstruct, but did advanced the cause of justice."

Flood's press aide said last night that congressman was not aware of any such investigation and would decline comment on his involvement in Elko's legal problems because of reports that he was a subject of other investigations.

In extensive interviews over the past two days, Ingoldsby detailed his involvement in Elko's legal problems. It began in November 1975 when a Senate investigating subcommittee subpoenaed the Flood aide to testify about alleged kickbacks for helping a California vocational school operator get federal funds.

The results of that investigation were refferred to the Justice Department and the House Committee on Standards of Official Conduct in the spring of 1976.

In April, Elko testified before a Los Angeles grand jury about the alleged trade school scheme after being briefed by Ingoldsby. Elko was later convicted of lying during that appearance. Ingoldsby said he was not aware Elko would commit perjury.

By May, Ingoldsby recalled, he had concluded it would be necessary for Elko to resign as Flood's administrative assistant to prevent a House investigation. "It was clear that the ethics committee had jurisdiction only to investigate current, not past employes," he said.

Shortly before the scheduled June 30 resignation lood and Ingoldsby met with Elko and his girlfriend, Patricia Brislin, who also was convicted on the West Coast bribery charges.

One account of the meeting is that Elko and Brislin strongly implied they might later implicate Flood if they were not "take care of" with help in finding employment and, if necessary, later legal fees.

Ingoldsby emphatically lenied that any such threats were made. "The main purpose of the meeting was for Patty (Brislin) and Steve (Elko) to make one final plea about another way of handing the matter."

He said he knew nothing about employment help and recalled no discussion of future legal fees.

Though there were other trials in the West Coast schools scandal later in 1976, Elko appeared free of legal difficulties until the following spring, the attorney said. Then, on June 9, 1977, Elko and Brislin were indicted Los Angeles for bribery, perjury and obstruction of justice for allegedly trying to prevent other testimony about their action.

Until then, Ingoldsby said, he had no intention of charging Elko and Brislin for his legal advise. "Steve had been very friendly to me, so I just let the thing ride," Ingoldsby said. I didn't send them amy bill. I had no intention of charging them."

But the day after the indictment he sent them a leter, pointing out that because of his close friendshop with Flood "it may be that a conflict of interest has "developed or will develop" between them and the congressman. He enclosed an unitemized bill for $10,000 in fees and $2,000 in expenses.

The couple replied that they appreciated his help and would pay the bill when they could, according to copies of the corresspondence made available by Ingoldsby.

"In all candor, there was a parallel interest between lawyer because "at first I thought it would end with the Senate, then I thought it would end with the House. . . ."

After the couple was arraigned in California, the new trial attorney, Alan M. May of Los Angeles, traveled to Washington to discuss his fee, Ingoldsby said.

May requested what Ingoldsby said he considered a "a bargain basement" $15,000 fee for handling the defense of the couple and $10,000 for expenses.

"I wanted to help them both," Ingoldsby said. "But now we were getting into an area where you have to be 100 percent correct on what's being done."

Without Flood's knowledge, Ingoldsby said, he called James J. Tedesco, a wealthy Flood friend who is a coal operator from Old Forge, Pa.

"I told him the situation, that these people said they were innocent, and we have to raise $25,000. He said he wasn't anxious to get involved, but would sent me a personal check for $15,000 to pay for the legal fees," Ingoldsby said.

"I said okay, it's either let them sink right now or I agree to advance the remaining $10,000. So I did," he added.

All the checks for disbursements for May's fees and expenses were written on the Ingoldsby office account, he said.

In a July 6, 1977, letter to Elko and Brislin explaining the arrangement, Ingoldsby said that he personally was advancing the entire amount.

Tedesco could not be reached for comment yesterday.

Flood was not informed of the arrangement, Ingoldsby said because "if he knew there would be no question that it would have the elements of a cover-up."

After the conviction in October, the attorney said, Elko told him that Justice Department prosecutors warned him "his troubles were just beginning."

THe former Flood aide was issued subpoenas for grand jury appearances in Philadelphia, Neward and Washington, Ingoldsby said.

After refusing to testify, Elko was given use immunity which compels testimony. He began cooperating with federal"authorities in early December.

Among the subjects he's being questioned on is the involvement of Flood and Rep. Joshua Eliberg (D-Pa.) in obtaining federal funds for an addition on aPhiladephia hospital project.

That investigation triggered a controversy when it was learned that Eilberg had called President Carter to urge the replacement of David W. Marston, the U. S. prosecutor in Philadelphia who was looking into the hospital financing. Marston has since left his post.

Ingoldsby said he first became aware he might have some legal problems when Elko called him early December to say he was being "grilled" by federal prosecutors.

Elko, who was sentenced last month to three years in prison on the bribery conviction, could receive a reduced sentence because of his cooperation.