(Reuters) - Authorities found no dangerous substances on Monday after investigating three suspicious envelopes sent to synagogues in Baltimore County, Maryland.

The incidents come amid a rise in hate crimes in the United States, with a 37 percent spike in anti-Semitic attacks, the third straight year that such attacks have increased, FBI data released last month said.

“The letters appear to be similar in nature and do not convey any type of threat,” Baltimore County said on its website.

Two adults working in the offices of the Beth El Congregation opened an envelope and immediately complained of feeling nauseous, the county said.

The letter sent to Beth El contained religious information about Jesus, according to the CBS affiliate in Baltimore.

Hazardous materials crews found no dangerous substances, the county said.

Officials later responded to the Beth Isaac Adath Israel Congregation because of a suspicious envelope, which was not opened, the county said. No dangerous substances were found.

The Har Sinai Congregation received a letter a week ago and reported it to police on Monday after learning about the other incidents.

It did not contain any hazardous substances.

Synagogues are on high alert after a gunman who had made anti-Semitic slurs on social media opened fire in a Pittsburgh synagogue on Oct. 27, one of the deadliest attacks on Jews in U.S. history.

Last week, an Ohio man, who said he admired the gunman in the Pittsburgh attack, was arrested and charged with planning an attack on another Jewish house of worship.