Trump was speaking with reporters outside the White House before leaving Washington for a rally in Wisconsin.
A gunman opened fire in the synagogue with an assault-style weapon on Saturday, killing one woman inside and wounding three other worshippers in a hate crime carried out on the last day of Passover, authorities said.
First Lady Melania Trump tweeted her condolence following the incident, citing the Passover holiday.
The suspect, a 19-year-old white male whose identity was not immediately made public, fled the scene by car and was arrested a short time later when he pulled over and surrendered to police, authorities said at a news conference.
The gun violence at the Congregation Chabad synagogue in the town of Poway, California, about 23 miles (37 km) north of downtown San Diego, unfolded six months to the day after 11 worshippers were killed and six others were wounded by a gunman who stormed a synagogue in Pittsburgh yelling, “All Jews must die.” The assailant in that incident was arrested.