Federal officials discovered Tuesday a poison-laced letter sent to Sen. Roger Wicker (R-Miss.), uncovering the material at an off-site location where congressional mail has been screened since anthrax-laced letters were sent to Capitol Hill in 2001.

Senate Majority Leader Harry M. Reid (D-Nev.) and other senators exiting an FBI briefing on the Boston attacks, told reporters that the letter was tested at the facility and came up positive for ricin. Officials gave no indication why the letter was sent to Wicker, a low-profile senator in his second term.

In 2004, three Senate office buildings were closed after preliminary tests found ricin delivered through the mail system in the Senate majority leader’s office. At the time the AP wrote, “Twice as deadly as cobra venom, ricin, which is derived from the castor bean plant, is relatively easily made and can be inhaled, ingested or injected.” But investigators later said the test may have picked up non-toxic byproducts of the castor bean plant used in paper production.

Several letters with white powder were delivered to Capitol Hill during the anthrax scare, and lawmakers, Obama and Cabinet secretaries remain a target of copycat attacks.

Once mail with federal addresses is sorted at a Washington facility, USPS trucks it to a New Jersey irradiation facility operated by Sterigenics, a company known for its medical sterilization equipment. The Postal Service said it spends about $12 million annually on irradiation but declined to comment further, citing security concerns.

But a 2008 Government Accountability Office report details what happens in New Jersey: The mail sent to federal addresses in Washington is heated to temperatures often exceeding 150 degrees. Large containers holding first-class mail and packages are scanned by a high-energy electron beam or X-rays to kill potentially harmful biological agents, including anthrax, the GAO said. The process delays the delivery of federal mail for two to three days.

The USPS irradiated about 1.2 million containers of government mail between 2001 and 2008, according to the GAO. That amount has dropped significantly in recent years with the government’s embrace of the Internet.