The Zika virus damages many fetuses carried by infected and symptomatic mothers, regardless of when in pregnancy the infection occurs, according to a small but frightening study released on Friday by Brazilian and American researchers.

In a separate report published on Friday, other scientists suggested a mechanism for the damage, showing in laboratory experiments that the virus targets and destroys fetal cells that eventually form the brain’s cortex.

The reports are far from conclusive, but the studies help shed light on a mysterious epidemic that has swept across more than two dozen countries in the Western Hemisphere, alarming citizens and unnerving public health officials.

In the first study, published in The New England Journal of Medicine, researchers found that 29 percent of women who had ultrasound examinations after testing positive for infection with the Zika virus had fetuses that suffered “grave outcomes.”