A little-known, decades-old measure keeping schools from using federal funds to bus students "in order to overcome racial imbalance" was included in the spending bill President Obama signed into law on Friday.

The measure in the 2,000-page legislation has been in spending bills since the 1970s, according to congressional and administration aides, but no one is sure of why the language is still included. And in a year fraught with racial tensions, some activists are taking notice.

The measure states: "No funds appropriated in this Act may be used for the transportation of students or teachers (or for the purchase of equipment for such transportation) in order to overcome racial imbalance in any school or school system, or for the transportation of students or teachers (or for the purchase of equipment for such transportation) in order to carry out a plan of racial desegregation of any school or school system."

The provision — along with other education-related amendments — was first approved when it was slipped into a spending bill in 1972. President Richard Nixon had failed earlier that year to get a standalone bill through Congress.

During that time period, after series of Supreme Court decisions, many school districts had mandated busing students in order to desegregate schools. In many parts of the country, elected officials were facing increasing pressure from white families — who opposed busing — to take action.

Decades later, aides and even members of Congress from both parties were unable to explain to BuzzFeed News why the provision is still included. Oklahoma Republican Rep. Tom Cole, a senior member of the House Appropriations Committee, said: "I'm not familiar with it at all to be honest with you. But every now and then we probably should weed the garden."

The provision was also included in President Barack Obama's budget request.

One of the leading figures in the Black Lives Matter movement Deray McKesson pointed out the language in the new spending bill on Twitter after it was first released on Wednesday.