Volunteering fell to the lowest rate in 2013 since the government has been measuring it, the Bureau of Labor Statistics reported Tuesday.

Roughly 62.6 million Americans volunteered through or for an organization at least once between September 2012 and September 2013, according to the BLS, or about 25.4 percent of adult Americans.

In 2002, the first time that the BLS conducted its survey on volunteerism, the rate was 27.6 percent. The decline last year was also a 1.1 percentage point drop from 2012.

Volunteerism has declined in each of the past three years. But it isn't clear whether the decline last year marks a broader trend away from volunteerism or an aberration. It also isn't apparent what might have caused Americans to volunteer less.

The drop can't be attributed to an improving labor market -- employed workers consistently volunteer at a higher rate than those out of work.

Although the decline may be related to an aging population, that trend is offset by the fact that the over-65 age group is the only one that has seen an increase in volunteering since 2002.

The turn toward slightly lower participation in volunteer activities is most pronounced among white and black Americans, and Hispanics have increased their participation in each of the past four years.

Women are still significantly more likely to volunteer than men, with a 28.4 percent participate rate versus only 22.2 for men.

Religious activities remain the main outlet for service, with volunteers contributing a third of their time to church-related efforts. The top activities were prepping and serving food, fundraising and tutoring.

The BLS' report on volunteerism was a supplement that went with its monthly Current Population Survey to 60,000 households. It was sponsored by the Corporation for National and Community Service.