A US politician has written an open letter to Facebook and Instagram asking them to stop gun and ammunition sales being arranged through the two sites.
Senator Edward Markey wants the companies to explain why he was able to find weapons being advertised on both social media platforms.
It's after a recent ban on users selling weapons on Facebook.
The senator wrote the letter after reports Dallas shooter Micah Johnson used a gun bought through the site.
However, police in Dallas have told NBC News that the gun he used was an AK-74 assault-style rifle, not the SKS semi-automatic rifle he was reported to have purchased via a contact on Facebook.
"I remain deeply concerned that gun sales on Facebook and Instagram - or sales posted online but negotiated and concluded offline - may circumvent or violate state and federal laws, resulting in numerous unlawful sales of handguns, assault weapons and other firearms," wrote Senator Markey.
"We want all communities, whether online or offline, to be safe for their members.
"I continue to urge Facebook and Instagram to adopt safe business practices and prohibit postings for firearms sales."
Facebook and Instagram banned the sale of weapons by individual users in January, although companies which sell guns are still allowed to advertise.
The two social networks had already banned the sale of guns without identity checks.
The Democrat senator for Massachusetts has given Facebook CEO Mark Zuckerberg and Instagram chief Kevin Systrom two weeks to reply to his letter, although they don't have to reply to him.
Edward Markey is also behind a proposed bill to stop under-16s in America from having or firing machine guns and assault weapons.
"The small hands of children and big power of machine guns are a deadly combination," he said in a statement.
"Assault weapons are weapons of war and should not be allowed in the hands of children."
The ban would include blocking children from firing the weapons at gun shows, gun ranges and while out hunting.