The body of a man in his forties found in the Charles River near Downtown Boston and the Esplanade early Monday morning has been tentatively identified, but authorities are withholding the decedent’s name pending contacting his family, since there was no evidence of foul play, a spokesman for the Suffolk County District Attorney’s office said yesterday.

Although the body was recovered by  the Boston Fire Department responding to a call from a passerby, the State Police detective unit from the Suffolk County District Attorney’s office is handling the investigation. Under Mass. law, State Police under the state’s various District Attorney’s Offices are responsible for death investigations occurring in the state waterways within their jurisdictions.

There was no indication on the body or in the vicinity of foul play, said Jake Wark, the spokesman for the Suffolk County District Attorney’s Office. The body, which did not appear to have been in the water for an extended period of time, showed no signs of trauma.

An autopsy did not disclose any sign of hidden trauma or internal injuries, Wark said. Results from a further autopsy, which will include toxicology and other specialized testing, could take several weeks or even a few months to be gathered.

Bodies are found in the Charles several times a year.

“It happens more than I once would have thought,” Wark said.

“Frequently they are men and women who are homeless. Occasionally they are the result of suicide attempts,” Wark said. “It’s rare that foul play is involved, but we do investigate each case in the event that a crime has been committed.”

Water quality in the Lower Basin, the area downstream of the Watertown Dam where the body was found, is generally good compared to other parts of the Charles, according to Julie D. Wood, a Watershed scientist at the Charles River Watershed Association (CRWA).

The appearance of a body in the water should not have a major impact upon water quality, Wood said.

“It’s an isolated incident...if you think of [the body] as a pollution source, it’s relatively small compared to the size of the River.”

Though the body may not affect water quality, swimming in the River is still discouraged for the majority of the year. In the 2009 the Lower Basin met the Massachusetts swimming standard for about half of the dry season. All samples from the area met less strict boating standards.

Citizens can find current water quality data on the CRWA’s website.

—Staff writer Michelle B. Timmerman can be reached at mtimmerman@college.harvard.edu.