From the Summary: "Attempt is the incomplete form of some other underlying offense. Unlike state law, federal law does not feature a general attempt statute. Instead, federal law outlaws the attempt to commit a number of federal underlying offenses on an individual basis. Occasionally, federal law treats attempt-like conduct as an underlying offense; outlawing possession of drugs with intent to traffic, for instance. One way or another, it is a federal crime to attempt to commit nearly all of the most frequently occurring federal offenses. Attempt consists of two elements. One is the intent to commit the underlying offense. The other is taking some substantial step, beyond mere preparation, collaborative of the intent to commit the underlying offense. The line between mere preparation and a substantial step can be hard to identify. Some suggest that the more egregious the underlying offense, the sooner preparation will become a substantial step."
CRS Report for Congress, R42002
Congressional Research Service: https://crsreports.congress.gov/