A strong earthquake shook Costa Rica on Thursday, killing at least three people and sending frightened residents running into the streets of the capital as windows shattered and walls cracked. The U.S. Geological Survey said the 6.1-magnitude temblor was centered 22 miles (35 kilometers) northwest of the capital of San Jose in the early afternoon, near the Poas Volcano national park. The National Emergency Commission said in a statement that two sisters, ages 7 and 11, were killed by a landslide unleashed by the quake near the city of Alajuela, north of the capital. In addition, a woman died of a heart attack in the capital, the commission said. Today is a day of mourning for Costa Rica because the earthquake killed three people,'' President Oscar Arias said. The commission said more than 200 people were injured and that 42 communities suffered damages. Some towns, including a mountain community popular with tourists, were isolated after landslides cut off roads, the agency said. Local TV station Teletica showed broken windows, fallen ceiling panels and cracked walls in Alajuela. The quake was felt strongly throughout the small Central American country, Teletica said. Cell phone service was interrupted briefly. Thousands of people ran from homes and shopping centers, frightened by the quake. In the capital, women kneeled to pray in plazas. Others cried. There were at least four aftershocks.