WASHINGTON — Violent and property crime both plunged across the United States in the first six months of 2020 as the coronavirus outbreak swept the country, the Federal Bureau of Investigation reported Tuesday.

Even though lockdowns to prevent the spread of Covid-19 were inconsistent and non-existent in some areas, murders fell 14.8 percent from a year earlier and rapes dropped 17.8 percent, according to preliminary data compiled by the FBI.

Violent robbery fell 7.1 percent, and non-violent thefts and larceny fell by slightly more from the first half of 2019, the FBI said.

But arson jumped in the first half of this year, especially in large cities and in West, it said.

Arson cases rose more than 52 percent in cities with populations over one million, and were up 28 percent in the western part of the country.

The FBI did not offer any explanation of the decline in crime overall, or the surge in arson.

But the period covered by the data coincides with the country’s response to Covid-19, including the declaration of a national emergency on March 13, California’s stay-at-home order on March 19, and New York issued a stay-at-home order on March 20.

Violent crime of all types fell in the period by 4.8 percent in the northeast and by smaller levels in the West and Midwest.

But violent crime increased compared to 2019 in the South, by 2.5 percent. Generally southern states lagged others in taking serious steps to prevent the spread of the coronavirus.

In campaigning for reelection, President Donald Trump has repeatedly alleged that crime has risen in cities led by Democrats, which includes many large cities like New York and Chicago.