Fairfax County prosecutor Robert F. Horan Jr. said yesterday he is investigating allegations that a county jail prisoner helped construct a shed at the home of county Sheriff James D. Swinson and in return received glowing recommendations from deputy sheriffs at a sentence-reduction hearing.

Ronald Lawhorne, who is in prison for killing his wife, Carol, testified yesterday in Fairfax Circuit Court that he had done work at Swinson's new Great Fals, Va., home last September. Lawhorne said he poured cement to build a 12-by-16-foot tool shed.

Swinson said yesterday that the work was done by John O. Feehan, the head jailer, and Feehan's uncle on their own time. Swinson said he doesn't know whether Lawhorne or any prisoner helped build his shed because he wasn't there all the time during construction.

"I don't know anything about it," Swinson said. "If the guy (Lawhorne) was over here it was unbeknownst to me. . . "

Horan said that according to Lawhorne he, another prisoner and Feehan went to Swinson's home to lay cement. During the hearing yesterday on the length of Lawhorne's sentence, the inmate took the Fifth Amendment when asked if he had done work on a private residence, Horan said.

When Judge William G. Plummer told him to answer the question, Lawhorne replied that he had worked at Swinson's home, Horan said. Lawhorne had been a foreman at a construction company before his conviction, Horan said.

Horan said that Feehan denied the allegations in testimony yesterday.

Horan said if Lawhorne's story is accurate, he would investigate bringing charges against someone in the sheriff's department because using a prisoner to do personal work is against the law.

Horan said the subject first came up in December when Judge Richard J. Jamborsky reduced Lawhorne's sentence by 2 1/2 years at a sentence-reduction hearing.

During the December hearing several sheriff's deputies testified in Lawhorne's favor. After the hearing a deputy told the judge that he had not heard the whole story, Horan said.

The hearing yesterday was to determine whether Lawhorne's sentence should be returned to its original eight-year duration. But Jamborsky decided to leave the sentence at the lesser number of years. Horan said.

Swinson said he doesn't know why the deputies would testify so strongly in Lawhorne's favor at the sentence-reduction hearing except that Lawhorne "is a hard-working guy. He is a handyman. He is a good man." Swinson said he had Lawhorne perform some work at the new jail.

Swinson said he first heard about the allegation three months ago. He met with several of his employees and each one said he knew nothing about the rumor, Swinson said.

"These things float around all the time," Swinson said.

Swinson said he had no way of knowing who was doing work at his house. Swinson also said he paid Feehan for the work and for the cement, but that he received a refund from Feehan for the money to buy the cement.

"As to who was out here and poured it, I don't know," Swinson said.

Swinson is a Republican who was elected sheriff in a post that primarily involves running the jail and serving court papers. Horan, Fairfax County's commonwealth's attorney, is a Democrat.