PHILADELPHIA (AP) _ An FBI agent reportedly under investigation by a federal grand jury looking into mortgage fraud has been fired, FBI officials said Tuesday while refusing to say why.

Frank R. Olivis Jr., assigned to investigate government fraud, was dismissed Monday, said John L. Hogan, special agent in charge of the Philadelphia division.

''He's been dismissed from the rolls,'' Hogan said. ''The matter has been referred to the appropriate U.S. attorney.''

The Philadelphia Inquirer reported Tuesday that Olivis is under investigation by a federal grand jury in Camden, N.J., that is looking into mortgage fraud.

Hogan, citing federal privacy laws, said he could not discuss the situation, other than to say it resulted from an internal investigation. He also said federal law prevented him from giving out Olivis' age and years of service in the FBI, or the name of his attorney.

Olivis' telephone number was not listed.

The Inquirer said the grand jury is reviewing evidence that could implicate Olivis in the crime of filing falsified documents to get a federally insured mortgage for his Burlington County, N.J., home.

Olivis' name surfaced last year when 22 investigators from the FBI, the Veterans Administration, the U.S. Department of Housing and Urban Development and the IRS in Camden investigated defaults and foreclosures on federally insured mortgages approved by HUD's Camden office, the newspaper said.

Two homes once owned by Olivis and his wife in Burlington County had foreclosure proceedings lodged against them over the past 31/2 years, according to property records.

The grand jury investigation, which began in January 1983, has led to indictments against three officers of a real estate company on charges of fraudulently obtaining $1.5 million in HUD-backed mortgages.

Federally insured mortgages require a small down payment, offer low interest rates and guarantee the mortgage will be paid by the government if the homeowner defaults.

Federal investigators who are not members of the Camden team told the Inquirer that Olivis came under investigation after foreclosure on a Veterans Administration mortgage to Olivis and his wife in 1981 surfaced during the inquiry and an agent recognized Olivis' name.