A letter sent to president Barack Obama apparently contained poisonous ricin, the FBI has revealed..
It is undergoing further testing because preliminary field tests can be unreliable, creating false positives.
The letter was intercepted away from the White House. It comes the day after officials said a letter sent to a US senator contained ricin. That was intercepted at a Senate mail facility just outside Washington.
The FBI said there was no indication of a connection to the bombing at the Boston marathon.
Meanwhile, the third person killed in the Boston Marathon bombings was a Chinese graduate student at Boston University who was originally from China's northeastern city of Shenyang, a state-run Chinese newspaper reported today.
The Shenyang Evening News said on its official Twitter-like microblog account that the victim's name is Lu Lingzi. An editor at the newspaper said Ms Lu's father confirmed his daughter's death when reporters visited the family home.
The Chinese Foreign Ministry and Consulate General in New York are not releasing the victim's name at the request of the family. But yesterday Boston media quoted a Chinese Consulate General official as saying that Chinese national Lu Lingzi was missing in the wake of Monday's bombings which killed three people and injured more than 170.
In the Chinese-language world of social media, people have been sharing their condolences on what is believed to be Ms Lu's microblogging account hosted by Sina Weibo, which was last updated on Monday with a breakfast photo. By early this morning, more than 14,000 comments had been left on the page.
Friends contacted through Sina Weibo have largely declined to speak to media about Ms Lu, saying they were adhering to the wishes of her family.
Ms Lu graduated from a Shenyang high school and studied international trade at Beijing Institute of Technology before going to the United States to study statistics as a graduate student at Boston University, according to media reports, Ms Lu's friends and her own Facebook page.
The Chinese form the largest contingent of foreign students at US colleges and universities. Last year, nearly 200,000 Chinese were enrolled in US higher education institutions, and Massachusetts had almost 10,000 Chinese students on its college campuses, according to the Institute of International Education.