The Thai security officials said on Tuesday that the victim was killed when gunmen fired shots at a bus and a flatbed truck carrying demonstrators.
"According to our initial intelligence information, it definitely involved politics," said Paradorn Pattanatabut, a security adviser to Prime Minister Yingluck Shinawatra, adding, "It is hard to control the situation."
Protesters were attacked while returning to their encampment from a demonstration at government offices.
Tensions in Thailand have once again escalated since anti-government protesters took back to the streets to pressure Premier Yingluck to resign.
The violence has left at least 24 dead and hundreds injured since November.
Thailand held a general election on February 2 in an attempt to curb anti-government protests, which were triggered in November 2013. However, on March 21, Thailand’s Constitutional Court ruled the elections disrupted by anti-government protesters invalid.
The protesters say Yingluck should step down so that an unelected “people’s council” could enact reforms.
Opponents also see the premier as a proxy for her elder brother, Thaksin Shinawatra, a former prime minister toppled in a coup in 2006. The ex-premier has been in self-exile since 2008 to avoid a two-year prison sentence.