DETROIT – The county prosecutor said Friday she has opened an investigation into reports that Mayor Kwame Kilpatrick and his top aide exchanged romantic text-messages.

The messages raise the question of whether the mayor and Chief of Staff Christine Beatty committed perjury during a whistle-blower trial last summer. A conviction of lying under oath can bring up to 15 years’ imprisonment.

Prosecutor Kym Worthy didn’t elaborate on the specifics of her investigation, but said she was unaware of the text messages until she read a report in the Detroit Free Press on Thursday.

“The Wayne County prosecutor’s office will conduct an independent investigation that will be fair, impartial and thorough,” Worthy said. “We will not be rushed by anyone or anything.”

Neither Kilpatrick nor Beatty planned on Friday to release any statements regarding the announced investigation, said mayoral spokesman James Canning.

A message seeking comment also was left on Beatty’s home phone.

Kilpatrick and Beatty testified last summer in a police whistle-blower lawsuit and denied any sexual or romantic ties in 2002 and 2003. But the Detroit Free Press examined about 14,000 text messages on Beatty’s city-issued pager from those years that tell a different story.

They reveal the two carried on a flirty, sometimes sexually explicit dialogue about where to meet and how to conceal their numerous trysts.

“I’m madly in love with you,” Kilpatrick wrote on Oct. 3, 2002.

“I hope you feel that way for a long time,” Beatty replied. “In case you haven’t noticed, I am madly in love with you, too!”

On Oct. 16, 2002, Kilpatrick wrote Beatty: “I’ve been dreaming all day about having you all to myself for 3 days. Relaxing, laughing, talking, sleeping and making love.”

Kilpatrick is married with three children. Beatty was married at the time and has two children.

A union leader called Friday for Kilpatrick to resign, saying the city could no longer believe what he says. John Riehl, president of American Federation of State, County and Municipal Employees Local 207, said the union wants the mayor to step down right away.

“He’s compromised the public trust,” Riehl said. “Nobody can believe a single thing he says from now on.”

The union has 900 members and represents workers in the city’s water and public lighting departments. The city has nearly 15,000 employees on its payroll, Canning said.

The Free Press did not explain exactly how it obtained the messages. The newspaper said it cross-referenced the messages with the mayor’s private calendar and credit card records to verify events in some of the notes.

A county jury awarded $6.5 million to the two officers in the lawsuit against the city and Kilpatrick. The jury said Kilpatrick and the city unlawfully dismissed two officers, who claimed they suffered after investigating allegations of wrongdoing within Kilpatrick’s security unit.

The events happened in Kilpatrick’s first four-year term. He was elected to a second term in November 2005.