The paper presents an inexact factorization technique in the context of an interior point method for optimal control of systems described by Differential Algebraic Equations (DAEs). The proposed method is based on a numerically banded structure arising in the normal equations. The structure implies that banded factorization techniques can be used to calculate inexact Newton directions at a low computational cost. Numerical experiments show that a narrow band in the normal equations can be selected without impeding the convergence of the method. (C) 2016 Elsevier Ltd. All rights reserved.