More than 30 others were injured when a bomb was set off early on Thursday at the car park of the prosecutor general's office in Medellin.
The suspect was captured at a barber's shop while dyeing his hair a different colour.
Colombian police blamed left-wing rebels from the Revolutionary Armed Forces of Colombia (FARC) for the attack, saying it was in retaliation for a security forces operation this week.
Medellin police chief Leonardo Gallego Castrillon has offered more than $170,000 for information leading to the arrest of those responsible.
The city has in the past been the scene of violence between Colombia's left-wing rebels and right-wing paramilitaries.
Security operation
The bomb attack came after troops and police conducted house-to-house raids in poor neighbourhoods of Medellin in recent days, arresting more than 60 suspected FARC rebels.
Last week four people died in a car bomb explosion in eastern Colombia.
Thousands of people - mainly civilians - are killed each year in Colombia's long-running civil war.
President Alvaro Uribe - a former mayor of Medellin - has vowed to defeat illegal armed groups.
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