美国马拉松爆炸：奥巴马发誓找出肇事者

　　WASHINGTON - US President Barack Obama, responding to the explosions at the Boston Marathon, says the United States does not know "who did this or why" but vowed¹ that whoever is responsible "will feel the full weight of justice."

　　He said: "We will find out who did this and we will hold them accountable."

　　US President Barack Obama pauses while he makes a statement about the bomb blast at the finish line of the Boston Marathon while in the Brady Press Briefing Room at the White House in Washington, April 15, 2013. [Photo/Agencies]

　　Obama made his remarks Monday evening from the White House about three hours after two explosions detonated near the marathon's finish line. At least two people were killed and 50 injured in the blasts.

　　Obama has been in touch with federal law enforcement and Massachusetts officials in the aftermath of the explosions.

　　The Secret Service reacted cautiously to the blasts, expanding the security perimeter² around the White House.

1 vowed   

起誓，发誓（vow的过去式与过去分词形式）

参考例句：

• He vowed quite solemnly that he would carry out his promise. 他非常庄严地发誓要实现他的诺言。
• I vowed to do more of the cooking myself. 我发誓自己要多动手做饭。

2 perimeter   

n.周边，周长，周界

参考例句：

• The river marks the eastern perimeter of our land.这条河标示我们的土地东面的边界。
• Drinks in hands,they wandered around the perimeter of the ball field.他们手里拿着饮料在球场周围漫不经心地遛跶。