The FBI said it obtained a swab used to stop the bleeding and found that DNA matched Tucker's DNA on other evidence from a 2009 robbery at Monroe County Community Credit Union in Temperance, near the Michigan-Ohio border.
In a court filing, agent Robert Schmitz said he was aware of Tucker's bout in April in Toledo, Ohio, and obtained the "discarded" Q-tips swab.
Tucker's DNA matched DNA from a mask believed to have been used in the robbery and from the steering wheel of the getaway car, the FBI said.
Tucker, 32, of Toledo, was arrested this week and ordered held without bond Wednesday. The government alleges he wore a mask and had a handgun during the $5,400 robbery. Another suspect was indicted in the case in November.
Detroit FBI spokesman Simon Shaykhet declined to discuss how Schmitz got the bloody swab. Defence attorney Haytham Faraj said there seems nothing illegal about acquiring it.
"We leave our fingerprints, bits of hair and skin all over the place. If you're a boxer, sometimes you leave your blood around," Faraj said in an interview Thursday.
He declined to comment on Tucker's alleged role in the robbery but doubts the blood sample really was necessary based on other DNA evidence.
"It is a dramatic twist. It makes for an interesting read," Faraj said.
He said Tucker, a light welterweight, is a roofer and part-time boxer. He's 1-6 in his last seven bouts, according to boxrec.com.
Bronco McKart, who also boxed that night in Toledo, knows Tucker and was surprised by the allegations.
"That's 'CSI Miami' stuff," McKart told the Monroe Evening News, referring to the bloody swab. "That's just amazing."