Manorville carpenter John Bittrolff, center, stands with his lawyer William...

Manorville carpenter John Bittrolff, center, stands with his lawyer William Keahon at his 2014 arraignment in state Supreme Court in Riverhead in the killings of two women 20 years earlier. Credit: James Carbone

The day after Suffolk’s chief medical examiner used sperm density to explain to a jury how soon after sex two women were killed, he conceded during cross-examination Tuesday that numerous factors could affect the reliability of his estimate.

Dr. Michael Caplan testified during the second-degree murder trial of John Bittrolff, 50, a Manorville carpenter accused of strangling and bludgeoning Rita Tangredi, 31, of East Patchogue on Nov. 2, 1993, and Colleen McNamee, 20, of Holbrook on Jan. 30, 1994. Both women worked as prostitutes, and Bittrolff’s DNA was recovered from semen found in both bodies.

Caplan earlier had testified that the density of sperm cells he found in different areas told him how long before death they could have been left there. He estimated that Tangredi was killed less than 26 hours after sex and that McNamee likely died less than 24 hours afterward.

During cross-examination by defense attorney Jonathan Manley of Hauppauge, Caplan said he had never before testified as an expert about drawing conclusions from sperm density.

Caplan agreed with Manley that such an analysis can be affected by who examined the microscopic slides and whether the slides were prepared properly. In this case, the slides were prepared in 1994 and Caplan said he did not know how that was done.

“But that does not mean the results are completely invalidated,” he said.

Manley also pointed out that some of the terms Caplan used in his analysis were imprecise. Caplan said people could interpret his use of undefined words like “many” or “some” differently.

Caplan also agreed with Manley that sperm density depends in part on how much is left in the first place, and men’s sperm counts vary.

During the trial in Riverhead, the prosecution has argued that because both women were killed in similar ways and posed similarly, they likely were killed by the same person. But during questioning by Manley, Caplan said there were differences between the two killings.

They include:

— Tangredi’s body was covered with leaves and brush; McNamee’s was not.

— McNamee’s right hand had possible cigarette burns, while no such injury was on Tangredi’s body.

— No bones were broken in Tangredi’s face, but every facial bone other than McNamee’s jaw and nose were broken.

— Tangredi’s skull cracked almost in two with a long, linear fracture. McNamee’s, on the other hand, was partly crushed like an eggshell.

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