WASHINGTON, March 15 – The Department of Homeland Security, trying to focus antiterrorism spending better nationwide, has identified a dozen possible strikes it views as most plausible or devastating, including detonation of a nuclear device in a major city, release of sarin nerve agent in office buildings and a truck bombing of a sports arena.

The document, known simply as the National Planning Scenarios, reads more like a doomsday plan, offering estimates of the probable deaths and economic damage caused by each type of attack.

They include blowing up a chlorine tank, killing 17,500 people and injuring more than 100,000; spreading pneumonic plague in the bathrooms of an airport, sports arena and train station, killing 2,500 and sickening 8,000 worldwide; and infecting cattle with foot-and-mouth disease at several sites, costing hundreds of millions of dollars in losses. Specific locations are not named because the events could unfold in many major metropolitan or rural areas, the document says.

The agency’s objective is not to scare the public, officials said, and they have no credible intelligence that such attacks are planned. The department did not intend to release the document publicly, but a draft of it was inadvertently posted on a Hawaii state government Web site.

By identifying possible attacks and specifying what government agencies should do to prevent, respond to and recover from them, Homeland Security is trying for the first time to define what “prepared” means, officials said.