JOHNSTOWN — Rodney L. Zobrist of St. Johnsville is already a convicted rapist, but Fulton County authorities, presented with DNA evidence linking him to two new rapes, are now calling him a serial rapist.

The 64-year-old Level III sex offender, already in jail on the basis of DNA evidence linking him to a 2007 rape in Ephratah, was arrested again Thursday when investigators discovered Zobrist’s DNA also made him a suspect in a 2005 Ephratah rape.

Fulton County District Attorney Louise K. Sira said Zobrist is a serial rapist. She said he has been convicted of two rapes and is now facing allegations in the two new cases.

He was arrested in May on charges he raped a 51-year-old woman on County Highway 119 while making a stop at her residence on the pretext of delivering a package, investigators said.

In this latest case, Sira said he gave a 17-year-old girl a ride in 2005 while she was walking on County Highway 119 in Ephratah and took her to a remote location where he forced himself upon her on a dirt road.

He is charged with first-degree rape in the 2005 and 2007 incidents, both of which Sira described as forcible. The Class B felony carries a maximum sentence of 25 years in prison.

He was classified a Level III sex offender on the basis of a 1992 rape in which he attacked a 20-year-old Fulton County woman, Sira said.

He was arrested in 2002 on charges he raped a 14-year-old girl in 1998 in St. Johnsville. Details on the disposition of that case could not be obtained late Thursday.

Sira, commenting after the May arrest, said that while there were opposing accounts of what transpired at the home of the 2007 victim, the DNA testing corroborated the victim’s version of events.

Sira said the 2007 victim called authorities immediately and was treated at a hospital for various injuries and screened for evidence of rape.

Three days elapsed before the 17-year-old reported the 2005 incident, but Sira said that DNA was recovered from her clothing.