。
She was caught red-handed trying to steal a necklace.
Literal
She [topic-は] necklace [object-を] try-to-steal [in the act of-て] flagrant-crime [in/on-で] was-caught.
盗もうとして combines the volitional 盗もう ('try / will steal') with として ('attempt to') — the pattern [volitional]+として means 'try to do, be in the act of doing.' Caught at the moment of attempting, before completion. 現行犯 (げんこうはん) is a legal/journalistic term for 'a crime committed in flagrante,' literally 'currently-active offence'; being arrested 現行犯で is the Japanese equivalent of 'caught red-handed.' The で here is the situational で, framing the manner of arrest. Direct passive つかまった ('was caught') by an unmentioned agent (presumably the police or store staff).