to be hot on the trail of someone or something = very close behind someone or something, and about to catch them
a suspect = someone the police believe may have committed a crime
In the cartoon you see a caricature of the Attorney General of the United States, Eric Holder. His position in the US government is the French equivalent to something between procureur général, ministre de la justice and garde des sceaux (but without any control over courts and judges). He is the main legal advisor in the government. The judicial system is separate and independent in the US.
Grammar note for 'attorney-general': This is one of those rare occasions when an adjective in English comes after the noun. The plural form of this word is attorneys-general. Other examples would be terms such as mother-in-law becoming mothers-in-law.
For my students: What message or idea is the cartoonist trying to convey?