David Hogg, one of the student leaders to emerge from the school shooting in Parkland, Fla., is calling on investors to boycott BlackRock Inc. and The Vanguard Group, the nation’s two largest investment-management companies, over their holdings in gun companies.

“[BlackRock] and [Vanguard] are two of the biggest investors in gun manufacturers; if you use them, feel free to let them know,” Hogg said Tuesday in a series of tweets, with the hashtags #BoycottVanguard and #BoycottBlackRock.

BlackRock BLK, -0.32% is the largest institutional shareholder of gun maker Sturm Ruger & Co. RGR, +4.15% and the second-largest investor of Vista Outdoor Inc. VSTO, +13.03% and Smith & Wesson Corp. parent American Outdoor Brands Corp. US:AOBC, according to Morningstar data.

Vanguard is the second-largest shareholder of Sturm Ruger, and third-largest of American Outdoor Brands and Vista Outdoor.

There’s a reason the two are so heavily invested — as so-called passive investors, they have to buy every stock in the index they track. As the popularity of passive investing has soared, the two fund managers have become giants.

See also: Are you invested in gun stocks? Read this if you want to change that

Hogg, 18, led a recent national advertising boycott of Laura Ingraham’s show on Fox News. He is a survivor of February’s massacre at Marjory Stoneman Douglas High School that left 17 people dead, and has become a vocal anti-gun advocate in the weeks since, with more than 760,000 Twitter followers.

BlackRock last month stripped retailers that sell guns from some of its exchange-traded funds, and said it would offer two new ETFs that exclude gun makers and retailers.

Aside from offering the FTSE Social Index Fund, which excludes gun makers, Vanguard told CNN on Tuesday that 359 of its 388 funds do not directly invest in gun makers.

“Vanguard is taking action, meeting with the leaders of gun manufacturers and distributors. We want to know how they will mitigate the risks that their products pose and how they plan to help prevent such tragedies from happening again,” Vanguard said in a statement.

“We believe that when a business poses a risk to society, it can also pose a risk to investors. We are expressing this viewpoint directly with company leaders because we believe greater focus and transparency on these issues will ultimately benefit society and investors alike.”