Boston police today stepped up security around the city’s public schools after several schools received letters that contained “random and disjointed’’ references to the terror group Al Qaeda, the FBI, and Nazis, according to police and a law enforcement official briefed on the investigation.

The typewritten letters, while disturbing in tone, did not contain dangerous chemicals and also did not contain a specific threat against an individual school or the system as a whole, the official said. The official said the letters appeared to have been authored by the same person.

“They are all typewritten … talking about Al Qaeda, the FBI, and Nazis,’’ the official said, who described the content as “rambling.’’

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The official said nearly a dozen letters were sent to schools.

“The writer referred to the missives as ‘a letter to the American people,’” the official said. The writer also made references to “herbal bombs’’ and to “poisonous organs in animals.”

In a joint statement posted this afternoon, the Boston schools and Boston police said no dangerous substances were found when the letters were submitted to forensic testing.

“While we remain vigilant and are taking appropriate action, we reiterate there is no credible physical threat and the letters did not contain any substances,’’ the agencies said. “The safety of our students and school personnel is our top priority.’’

After the letters were received, staffers noticed the disturbing content they contained, immediately isolated them, and then notified Boston police, officials said. Now, all letters with a Texas address are being isolated and turned over to police.

Just after 2 p.m., the school department sent out an automated telephone call to parents and students alerting them to the situation.

Police spokeswoman Cheryl Fiandaca said police would increase their presence at schools around the city this afternoon.

This is the most recent in a spate of suspicious letters sent to various organizations and individuals. Last week, the Secret Service intercepted a letter containing traces of the poison ricin sent to President Barack Obama.

It was not known if there was a connection between the letters sent to the president and those received by the Boston schools.

City officials would not identify the specific schools involved.

Police said they are sharing information with the Joint Terrorism Task Force, which also played a role in the pursuit of Boston Marathon terror bombing suspects.

Boston Police Superintendent William Evans reiterated at an evening briefing that the letters contained no specific threat to the schools and no hazardous materials, and that authorities did not believe children were in danger.

He said the letters’ contents were identical.

“Based on what we clearly see, no one should feel that their child is in danger [by going] to school tomorrow,” Evans said.

He released few details about the typewritten letter’s contents but confirmed there was a reference to Al Qaeda, as well as “all kinds of weapons of mass destruction.”

“It just rambles on,” he said.

Evans added that the letters were not signed, and he could not say where in Texas they were postmarked from. He said beefed-up patrols would be posted outside schools on Friday.