The discovery was made on Stratford Avenue in the Soundview section of the Bronx.
Police Commissioner Ray Kelly said that officers detonated that grenade.
Investigators also found a second grenade in the process of being assembled and a stun gun.
Four people in the apartment were taken into custody for questioning.
Two people were later arrested.
24-year-old Anthony Cibelli and 25-year-old Juan Pagan were charged with Criminal Possession of a Weapon (Explosive Device), Criminal Possession of a Weapon (Firearm), Criminal Possession of a Controlled Substance and Criminal Possession of Marijuana.
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