FBI discovered cache of 25 guns after arresting a teenager who supported mass shootings.

FBI agents in the United States have discovered a cache of some 25 guns and 10,000 bullets after arresting a teenager who made online comments supporting mass shootings.

In a complaint filed Monday, the FBI charged Boardman, Ohio resident Justin Olsen with threatening to assault a federal law enforcement officer after he posted on an online forum: "In conclusion, shoot every federal agent on sight".

The 18-year-old's account on the iFunny website -- a forum where users share memes -- had approximately 4,400 subscribers.

He had previously made posts supporting mass shootings and targeting family planning and reproductive rights group Planned Parenthood, the complaint said.

After searching his father's house, agents found around 15 assault rifles and shotguns and 10 semi-automatic pistols inside the bedroom of Olsen's father, along with the ammunition.

Agents also found "camouflage clothing and camouflaged backpacks," according to the complaint.

Olsen admitted to making the online comments, the complaint said, but insisted they were "only a joke" and were made in regards to the controversial 1993 standoff in Waco, Texas.

The Branch Davidians religious sect held off federal authorities attempting to arrest their members on weapons charges, until the FBI eventually launched an assault and used tear gas. A fire engulfed the sect's compound and 76 people died.

Olsen's arrest came a week after back-to-back massacres killed 31 people in El Paso, Texas and Dayton, Ohio, sparking calls for lawmakers to limit the availability of firearms and reduce the carnage from gun violence in the United States.

President Donald Trump has said his fellow Republicans will set aside their longstanding resistance to restricting access to firearms by supporting background checks for people buying weapons, but his Democratic challengers in the 2020 presidential election have called for stronger measures.

Days after the killings in Texas and Ohio, a 20-year-old man wearing body armor, fatigues and armed with an assault rifle was arrested at a Missouri Walmart store -- the chain that was the site of the shooting in El Paso -- and charged with making a terrorist threat.