(CNN) -- Hate is alive and well in America. There are 784 hate groups nationwide, according to the Southern Poverty Law Center, and they are active in every state. When these groups commit hate crimes, they don't just terrorize individuals, they terrorize entire communities and can spark national outrage and unrest.

Yet since 2009 the federal government has obtained only 29 hate crime convictions. That's because a troubling legal interpretation has rewritten the federal hate crimes law and created a near insurmountable burden for federal prosecutors looking to obtain a hate crime conviction. Courts now require prosecutors to show that hate was the sole factor motivating the crime, not just a substantial motivating factor. Those who assault or kill with mixed motives are not guilty of a hate crime.

This disturbing reasoning can be extended in myriad ways, as mixed motives come in many forms. If a Christian attacks a gay man because he disapproves of his sexual orientation, but also dislikes the way he dresses and talks, it's not considered a hate crime. If a white man assaults a black woman because of her race, but also because she rejects his sexual advances, it's likewise not considered a hate crime.

As a civil rights lawyer in Washington, I've had numerous conversations with lawyers from the Department of Justice, and they regularly cite the difficulty of proving sole motivation as the greatest impediment to obtaining a hate crime conviction. Congress can remedy this failure by passing legislation clarifying that bias need only be a substantial motivating factor in proving a hate crime, not the sole factor.

America needs a strong and comprehensive federal hate crimes statute. Although 45 states have their own state hate crime laws, a robust federal law ensures that local prosecutors vigorously investigate and prosecute hate crimes. Local prosecutors are less likely to claim insufficient resources or turn a blind eye to hate-based violence if they know the federal government has the authority to step in and prosecute.

A strong federal law also enables federal prosecutors to bring cases that have national significance or when states don't have their own hate crime laws. The federal government's indictment of Dylann Roof in July 2015 in the deaths of nine African-Americans in Charleston, South Carolina, is illustrative.

Roof is accused of intentionally targeting the iconic Emanuel African Methodist Episcopal Church because of its local and historical significance to African-Americans. It was appropriate for federal prosecutors to investigate and try the case because of its national consequences. Moreover, because South Carolina is one of five states that doesn't have a hate crime law, the racial component of the crime would have gone unaddressed had the federal government not intervened.

Still, the onerous causation analysis imposed by courts means federal prosecutors can only go so far. Since the June 17 shooting in Charleston, for example, at least eight African-American churches in the South have been burned down in what appears to be a series of hate crimes intended to terrorize the African-American community. Yet, if the assailants acted not just out of racial hatred, but also economic consternation and political frustration, federal prosecutors would only be able to bring arson charges. Indeed, this is likely why no hate crime charges have been filed in these attacks.

This October marks the sixth anniversary of The Matthew Shepard Act, which amended the original 1969 U.S. federal hate crime law to include crimes motivated by gender, sexual orientation, gender identity, or disability, and gave states additional funding to investigate and prosecute hate crimes.

Now Congress must act again to ensure we have a strong and comprehensive federal hate crimes law, not one riddled by legal and technical machinations.

Editor's note: Arjun Singh Sethi is a writer, lawyer and director of law and policy at The Sikh Coalition. He is also an adjunct professor of law at Georgetown University Law Center. The opinions expressed in this commentary are solely those of the author.

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