Two people were killed and nine others were missing after a powerful explosion ripped through a chocolate factory in West Reading, Pa., on Friday, sending a plume of smoke into the air and shaking houses blocks away, officials said.

The explosion at around 5 p.m. destroyed part of the R.M. Palmer Company chocolate factory in West Reading, in Berks County, about 60 miles northwest of Philadelphia, Mayor Samantha Kaag said.

Wayne Holben, West Reading’s police chief, confirmed the number of dead and missing at a news conference on Friday evening.

The cause of the explosion was under investigation, he said.

Reading Hospital received a total of six patients, said a spokeswoman, Jessica Bezler. She said one of the patients would be transferred and the others were being evaluated.

R.M. Palmer, founded in 1948, employs 850 people, according to its website, and is known for making seasonal chocolates, including hollow milk chocolate bunnies for Easter.

The explosion sent a column of debris, flames and dust shooting into the air, as shown in a video that was captured by weather cameras and shared on Twitter by a reporter for Fox 29 News in Philadelphia.

“Part of the building is leveled,” Ms. Kaag said in an interview on Friday evening from the scene. She said she had felt the explosion at her house, four or five blocks from the factory.

“I didn’t hear a boom,” she said. “I just felt it shake my house.”