Investigators in San Antonio have begun poring over hospital records from the Bexar County Medical Center Hospital to determine the cause of a series of unexplained deaths of infants over several years.

A San Antonio grand jury already has heard from roughly two dozen witnesses during four days of testimony in the case.

The San Antonio investigators are cooperating with law enforcement officials in nearby Kerr County, where another grand jury is investigating the death of an infant in Kerrville, Tex., last fall. The number of infant deaths under investigation is not clear. Sources close to the probe say about 10 cases are under scrutiny in San Antonio. Published reports have indicated that more than 40 cases are being looked at, but officials in the Bexar County district attorney's office said today that they have not determined how large the case number will be.

The district attorney's office has subpoenaed records of all infant deaths at the medical center from 1978 to early 1982, according to one official.

"The bottom line is that we have just begun," said Luis Vallejo, who is heading the investigation. "It's going to take months. We will look at each and every death, even those that are allegedly explained."

The deaths occurred in the medical center's pediatric intensive care unit.

A number of the infants died of cardiac arrest, and investigators are examining whether drug injections caused the deaths.

Bexar County District Attorney Sam D. Millsap Jr. said the case has become the "No. 1 priority" of his office.

Investigators in San Antonio began to look into the fatalities last month, after being alerted to several suspicious deaths that occurred over several years. About the same time, they received information from the office of Kerr County District Attorney Ron Sutton that suggested a link between the death in Kerrville and those at Bexar County Medical Center.

The Kerr County grand jury has been meeting since last fall on the death and several unexplained infant illnesses.

The parents of the 15-month-old infant who died in Kerrville have filed a wrongful death suit in state district court, alleging negligence by Dr. Kathleen Holland, 36, and Genene Jones, 32, a licensed vocational nurse. Holland and Jones worked at Bexar County Medical Center Hospital at the time of the deaths now being investigated.

Jones appeared before the San Antonio grand jury Thursday and was cooperative, according to sources close to the case. "She was informed she was a focus in the investigation," one said.

Holland was subpoenaed to testify Thursday, but did not appear after her lawyer told the district attorney's office that she would invoke the Fifth Amendment to protect herself from self incrimination, according to published reports. Millsap met with hospital officials Thursday to discuss the results of an internal investigation into infant deaths at the medical center and to encourage them to help expedite the grand jury probe.

"We're continuing to cooperate fully," said Ross McLerran, assistant director of community relations.

Investigators said they would concentrate on evaluating the medical records for the infants who died at the hospital, while continuing to seek testimony of other hospital employes.

The San Antonio grand jury has scheduled no more testimony, but its term has been extended.