The phrase "attack letter" is correct and usable in written English.
It can be used to describe a letter that criticizes or attacks someone, often in a personal or aggressive manner.
Example: "The politician received an attack letter from an anonymous source, filled with accusations and personal insults."
Alternatives: "criticism letter" or "hostile letter".
All the attack letters contained these four morphs as well as the predominant form of Ames ancestor-type spores.
The F.B.I. made no mention of handwriting analysis, for example, that might tie Dr. Ivins to the attack letters, Dr. Tucker said.
Based on that interpretation, Barbara Hatch Rosenberg, a private expert in biological weapons at the State University of New York at Purchase, concluded in widely cited December report that the powdered anthrax in the attack letters "may be a remnant of the U.S. biological weapons program".
In the summary of its findings, the F.B.I. states that investigators used four different genetic techniques to match the anthrax-laced attack letters to a unique DNA footprint of a single anthrax spore preparation in one flask that had been in Dr. Ivins's custody.
Similarities between the strains used in Ivins's laboratory and in at least four attack letters were a key element in FBI's case, but a failure to understand how the bacteria mutate over time could complicate efforts to compare samples.
Anthrax entered the US national consciousness in 2001, when shortly after the 9/11 attacks, letters containing powdered anthrax arrived at news organisations and the offices of US senators.
Although it was written three years before the Cole attack, the letter's timing would fit al Qaeda's pattern of showing patience and long-term planning before conducting terrorist operations.
"The primary purpose of broadcasting to the outside world after a nuclear attack," one letter remarked, "would presumably be to let the world know that organised society still existed in the UK".
In their most recent attack, a letter bomb was delivered to the Institute of Biotechnology in the National Autonomous University of Mexico last month, but it was a dud – suggesting they are either adopting scare tactics or just getting worse at making bombs.
Los Angeles topped the list of cities based on dog attacks on letter carriers, with 69 attacks reported in fiscal 2012.
