The driver was Secret Service Agent William Greer.

Entity and relation annotations:
id: 0
type: entity
value: text: William Greer
labels: <Person>
id: 1
type: entity
value: text: Secret Service
labels: <Organization>
id: 2
type: entity
value: text: Agent
labels: <Title>
type: relation
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labels: Person:affiliated_with:Organization
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type: relation
to_id: 2
labels: Person:holds_title:Title
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type: relation
to_id: 2
labels: Organization:Establishes:Title
from_id: 1