Researchers who examined the lungs of patients killed by covid-19 found evidence that it attacks the lining of blood vessels there, a critical difference from the lungs of people who died of the flu, according to a report published Thursday in the New England Journal of Medicine.

Critical parts of the lungs of patients infected by the novel coronavirus also suffered many microscopic blood clots and appeared to respond to the attack by growing tiny new blood vessels, the researchers reported.

The observations in a small number of autopsied lungs buttress reports from physicians treating covid-19 patients. Doctors have described widespread damage to blood vessels and the presence of blood clots that would not be expected in a respiratory disease.