Courtesy IBEW Local 103

Less than 10 miles from the finish line of the Boston Marathon, where three people were killed and more than 170 injured in dual blasts on Monday, lies a billboard with a one-word message: "Cowards."

The billboard, located off the I-93 expressway that crosses the city, belongs to the local chapter of the International Brotherhood of Electrical Workers (IBEW) union. The chapter, Local 103, usually uses the billboard for union-related and community messages but that changed Tuesday morning, less than 24 hours after the two bombs exploded.

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Since then, the electronic billboard had rotated every 10 seconds between the "Cowards" sign and another that reads, "Pray for Boston."

"It was Local 103's way of shouting out our feelings of anger: Cowards," business manager Michael P. Monahan told ABCNews.com.

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None of the union's 7,500 members were directly hurt in the explosions, according to a representative, but they have another connection.

The town where the chapter's headquarters is located, Dorchester, Mass., is the same town where the blasts' youngest victim, Martin Richard, and his family lived. Richard, 8, died while attending the marathon festivities with his mother, father and two siblings. Both his mother and younger sister sustained "serious" injuries in the attacks.

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"No other word or words came to my mind when I heard of the tragic event downtown Boston on Patriot's Day when a family from our neighborhood was effected so tragically," Monahan said.

Local 103 plans to keep the billboard devoted to the Boston Marathon survivors but is working on incorporating new messages, the representative told ABCNews.com.