The 14th suspect arrested in connection with the brutal Bronx slaying of 15-year-old Lesandro "Junior" Guzman-Feliz pleaded not guilty Monday to a slew of charges.

Those charges against Frederick Then, 20, include murder, manslaughter, conspiracy, criminal possession of a weapon and two counts of gang assault. He is accused of acting in concert with the 13 men already arrested in Junior's slaying outside a bodega in the Belmont section of the Bronx. He's also a suspected member of the Trinitarios gang.

Police say Then turned himself in around 5 p.m. Sunday. He could face up to 25 years to life in prison if convicted of the top charge. Then joins 13 other suspected Trinitarios gang members who are awaiting trial on Rikers Island.

Prosecutors say the suspects mistook Junior for someone else on the night he was killed.

"This is so painful for me, this is so sad," says Leandra Feliz, Junior’s mom. "Because I was counting 14 people and my son was only 15 years old."

Then is being held without bail at the request of Assistant District Attorney Morgan Dolan.

"There is video surveillance capturing his pursuit on foot of Junior that ultimately led to Junior going into a bodega where he was pulled out and killed by members of the Trinitarios gang," Dolan says.

Prosecutors also believe Then, a Pennsylvania resident, is a flight risk, likely to leave the country before trial. The judge granted the request that Then may not be allowed to post bail.

All 14 suspects will be back in court on Oct. 25 for a pretrial conference between the prosecution and defendant's lawyers.