Context : Ultra-faint dwarf galaxies recently discovered around the Milky Way ( MW ) contain extremely metal-poor stars , and might represent the building blocks of low-metallicity components of the MW .
Among them , the Boötes I dwarf spheroidal galaxy is of particular interest because of its exclusively old stellar population .
Detailed chemical compositions of individual stars in this galaxy are a key to understanding formation and chemical evolution in the oldest galaxies in the Universe and their roles in building up the MW halo .

Aims : Previous studies of the chemical abundances of Boötes I show discrepancies in elemental abundances between different authors , and thus a consistent picture of its chemical enrichment history has not yet been established .
In the present work , we independently determine chemical compositions of six red giant stars in Boötes I , some of which overlap with those analyzed in the previous studies .
Based on the derived abundances , we re-examine trends and scatters in elemental abundances and make comparisons with MW field halo stars and other dwarf spheroidal galaxies in the MW .

Methods : High-resolution spectra of a sample of stars were obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope .
Abundances of 12 elements , including C , Na , \alpha , Fe-peak , and neutron capture elements , were determined for the sample stars .
The abundance results were compared to those in field MW halo stars previously obtained using an abundance analysis technique similar to the present study .

Results : We confirm the low metallicity of Boo-094 ( [ Fe/H ] = -3.4 ) .
Except for this star , the abundance ratios ( [ X/Fe ] ) of elements lighter than zinc are generally homogeneous with small scatter around the mean values in the metallicities spanned by the other five stars ( -2.7 < [ Fe/H ] < -1.8 ) .
Specifically , all of the sample stars with [ Fe/H ] > -2.7 show no significant enhancement of carbon .
The [ Mg/Fe ] and [ Ca/Fe ] ratios are almost constant with a modest decreasing trend with increasing [ Fe/H ] and are slightly lower than the field halo stars .
The [ Sr/Fe ] and [ Sr/Ba ] ratios also tend to be lower in the Boötes I stars than in the halo stars .

Conclusions : Our results of small scatter in the [ X/Fe ] ratios for elements lighter than zinc suggest that these abundances were homogeneous among the ejecta of prior generation ( s ) of stars in this galaxy .
The lower mean [ Mg/Fe ] and [ Ca/Fe ] ratios relative to the field halo stars and the similarity in these abundance ratios with some of the more luminous dwarf spheroidal galaxies at metallicities [ Fe/H ] < -2 can be interpreted as star formation in Boötes I having lasted at least until Type Ia supernovae started to contribute to the chemical enrichment in this galaxy .