The Department of Homeland Security confirmed last week that the highly contagious foot-and-mouth virus had briefly spread within the Plum Island Animal Disease Center in two previously undisclosed incidents earlier this summer. The incidents and their belated public disclosure raised new questions about laboratory safety and communications to the public. In a letter to the laboratory director, Beth Lautner, dated Aug. 2, Senator Hillary Rodham Clinton and Representative Timothy Bishop of Southampton said they were alarmed that the two incidents had taken place within a one-month span. A spokesman for the department, said the virus had remained within the laboratory's sealed biocontainment area. Asked why the department did not make a public announcement of the events, Mr. Tighe said: It was within the laboratory environment, safely sealed in biocontainment. This was really an operational issue." Plum Island is the only location in the United States where research on the foot and mouth virus is legally permitted. In 1978, a foot and mouth outbreak among animals in pens outside the laboratory resulted in new procedures for keeping animals used in research inside the biocontainment area. Since taking over the laboratory just over a year ago, Homeland Security had been emphasizing its intention to keep the public informed. But department officials apparently did not heed calls from elected officials to disclose the two incidents sooner.