  The reaction $\gamma p \to \pi^+K^-K^+n$ was studied at Jefferson Lab using a
tagged photon beam with an energy range of 3-5.47 GeV. A narrow baryon state
with strangeness S=+1 and mass $M=1555\pm 10$ MeV/c$^2$ was observed in the
$nK^+$ invariant mass spectrum. The peak's width is consistent with the CLAS
resolution (FWHM=26 MeV/c$^2$), and its statistical significance is 7.8 $\pm$
1.0 ~$\sigma$. A baryon with positive strangeness has exotic structure and
cannot be described in the framework of the naive constituent quark model. The
mass of the observed state is consistent with the mass predicted by a chiral
soliton model for the $\Theta^+$ baryon. In addition, the $pK^+$ invariant mass
distribution was analyzed in the reaction $\gamma p\to K^-K^+p$ with high
statistics in search of doubly-charged exotic baryon states. No resonance
structures were found in this spectrum.
