The United States Drug Enforcement Administration (DEA) is the federal agency tasked with enforcing controlled substances laws, including cannabis laws. Cannabis remains illegal at the federal level for now, which is important because that is the default rule for states that have not enacted more permissive laws. Federal illegality is also important even in states where cannabis has been partially or fully legalized because federal law still controls in airports, highways, governmental agencies, etc.
The Controlled Substances Act (CSA), enacted in 1971, regulates the distribution and use of several controlled substances, including cannabis. Cannabis remains a Schedule I substance under the CSA, meaning that it is not currently accepted by the federal government for medical or other use, and it is considered to have a high potential for abuse. Public access to the Act is linked above, and it is also available on Lexis and Westlaw.
The Agricultural Improvement Act of 2018 (Farm Bill) exempts from the CSA hemp products containing less than 0.3% of THC, meaning that non-THC hemp can be cultivated and sold legally. Public access to the Act is linked above, and it is also available on Lexis and Westlaw.
Other laws pertaining to cannabis would likely be found under Title 21 (Food and Drugs) , Title 7 (Agriculture), or Title 34 (Crime Control & Enforcement) of the U.S. Code. Public access to the Titles are linked above, and the U.S. Code can also be found on Lexis and Westlaw.
Cannabis regulations can be found in Title 21 (Food & Drugs), Title 7 (Agriculture), and Title 28 (Judicial Administration) of the Code of Federal Regulations (CFR). Public access to the Titles are linked above, and the CFR can also be found on Lexis and Westlaw.