The investigation into last week's mysterious kidnaping of the director of an Illinois abortion clinic and his wife expanded yesterday as authorities confirmed that a group called the "Army of God," which has claimed credit for the kidnaping, also said it was responsible for the May firebombing of two Florida abortion clinics.

Dr. Hector Zevallos, 53, and his wife, Rosalie Jean, 45, vanished from their home in Edwardsville, Ill., across the Mississippi River from St. Louis, on the night of Aug. 12. The investigation into their disappearance began the next day after Zevallos failed to appear for work at his Hope Clinic for Women in Granite City, Ill., not far from Edwardsville.

Authorities said that when they went to the Zevallos home they found the lights on, the television running, food on the table and no sign of a struggle. The doors were unlocked and the couple's two Mercedes Benz were in the driveway.

FBI sources said yesterday that the Army of God had not come to the bureau's attention until the Zevallos kidnaping.

But George Pinckney of the St. Petersburg, Fla., police department confirmed yesterday that a group calling itself the Army of God sent a letter to a local television station claiming responsibility for two firebombings of abortion clinics in the area on May 29.

The firebombings caused extensive damage to the St. Petersburg Women's Health Center and the Bread and Roses clinic in Clearwater, Fla.

"I had never heard of them before and I haven't heard from them since. I have no idea how many people are in this organization or who it might be," Pinckney said, adding that police have made no arrests in the two arson cases.

Last Jan. 23, the Zevallos clinic was also damaged in a fire of suspicious origin, but authorities apparently did not link that fire to any anti-abortion group.

Earlier this week, in a statement issued by the FBI and the Madison County Sheriff's Department, FBI agent Joseph Ondrula said the couple appeared to have been kidnaped by persons who left a "series of statements, including demands, labeled as epistles."

The kidnaping letter said, "in effect, that man is evil, man's institutions have become instruments of evil and are working to deprive man of life, liberty and prosperity in defiance of God's will."

The St. Louis Post-Dispatch reported that the letter did not contain a ransom demand but denounced abortion and criticized the government for allowing it. The letter demanded not only that abortion be stopped, but also that President Reagan denounce abortion and call for an end to the procedure, the newspaper reported.

Reagan, who has often spoken out against abortion, had no comment yesterday.

Meanwhile, Glenn Young, special agent in charge of the FBI's St. Louis office, made a plea to the kidnapers Wednesday night, saying, "We're running out of time," and asked the news media to publicize the request for the kidnapers to negotiate.

"The preservation of life is a concern of all of us," the FBI appeal said, "and therefore we wish to establish a contact, so that your cause can be furthered and human life can be spared."

The statement cautioned that "any adverse action" taken against the couple would be "detrimental to the cause to which you are committed."

Anti-abortion groups around the country have expressed surprise at the kidnaping. Gary Curran of the American Life Lobby said yesterday he has never heard of the Army of God. "They sound like a bunch of nuts," he said.

But the National Abortion Federation (NAF), which represents 240 abortion facilities in this country, said yesterday that in the past year there has been escalating violence against abortion clinics around the country.

Uta Landy, executive director of NAF, said there has been a notable increase in threatening telephone calls, picket lines and marches on clinics, harassment of staff and patients and threats of firebombings and bombings.

Landy said, "We trust the anti-abortion movement does not condone this kind of terrorizing, but we must recognize the fact that the fanaticism, the self-righteousness and the inflammatory language of the anti-abortion movement have set the stage for this kind of terrorism.

"It is no surprise that this would set off people who are fanatic. We are afraid. It is ironic to think this fear has been created by a movement that calls itself the right-to-life movement."