CHARLESTON COUNTY, S.C. (WCBD) – Charleston County is suing the South Carolina State Infrastructure Bank for breach of contract to complete Interstate 526.

The suit was filed Thursday with the S.C. Supreme Court, according to Charleston County officials.

The SIB promised $420 million to finish the Mark Clark Expressway a decade ago, but after delays and inflation, the project is expected to cost $725 million.

The bank voted in May of 2016 to unwind the I-526 project citing Charleston County’s inability to provide a firm plan to cover the more than $300 million shortfall.

Proponents for the completion of I-526 say the 8-mile long extension is necessary for safe evacuation routes and to alleviate traffic congestion in the area.

Critics say extending the highway into Johns Island will quash its quaint natural feel with prospect of new developments I-526 could bring.

I-526 is an interstate that ends at Highway 17 in Mount Pleasant and Savannah Highway in West Ashley

S.C. Department of Transportation’s conceptual route for completion of the highway begins where it currently ends in West Ashley. Their plans show it extending it across Johns and James Islands – meeting where the James Island connector and Folly Road intersect.